Run test program under xvfb-run to satisfy need for an X11 display (!49)

Running test_ext2 in GitLab Continuous Integration environment fails like
this:
    (test_ext2:6338): Gtk-WARNING **: 09:06:17.576: cannot open display:
    Running main() from test_ext2.cc

Obviously the GitLab CI environment doesn't have an X11 display, but
unfortunately this test case code requires one.
Utils::execute_command() calls Gtk::Main::run() so requires a Gtk::Main
object constructing and therefore an X11 display, even though this
program never displays anything graphical.  The call chain is:
    main()                       test_ext2.cc
      Gtk::Main::Main()          gtkmm/gtk/src/main.ccg
        Gtk::Main::init()        [1]
          gtk_init()             gtk/gtk/gtkmain.c [2]
which exits with a non-zero exit status when the DISPLAY environment
variable is unset.

Looked at deriving from Gtk::Main class and writing a replacement init()
method which calls gtk_init_check() instead of gtk_init() but
Gtk::Main::instance_ is a private member so not accessible in a derived
class.

Tried using Glib::MainLoop instead of Gtk::Main, but that doesn't
initialise everything that Gtk::Main(), so the program crashes.

Therefore use xvfb-run [3][4] to run this test program against a virtual
X11 display when a real display isn't available.  Coded execution of
xvfb-run into this test program so that it can simply be executed on the
command line like the other test programs, without having to remember to
run "xvfb-run ./test_ext2 ...".

[1] Gtk::Main::init()
    https://gitlab.gnome.org/GNOME/gtkmm/blob/3.10.1/gtk/src/main.ccg#L287
[2] gtk_init()
    https://gitlab.gnome.org/GNOME/gtk/blob/3.10.9/gtk/gtkmain.c#L1000
[3] how to run gtk app on linux without an x server
    https://superuser.com/questions/624918/how-to-run-gtk-app-on-linux-without-an-x-server
[4] Using GTK without DISPLAY
    https://stackoverflow.com/questions/11694278/using-gtk-without-display

Closes !49 - Add file system interface tests

.gitlab-ci.yml

@@ -11,7 +11,7 @@ stages:
                      gcc-c++ libuuid-devel parted-devel gtkmm30-devel make
                      polkit file
     # Extra packages only needed during the test stage.
-    - yum install -y e2fsprogs
+    - yum install -y e2fsprogs xorg-x11-server-Xvfb
     - cat /etc/os-release
 
 .ubuntu_image_template: &ubuntu_image_definition
@@ -23,7 +23,7 @@ stages:
                          uuid-dev libparted-dev libgtkmm-3.0-dev make
                          policykit-1
     # Extra packages only needed during the test stage.
-    - apt-get install -y e2fsprogs
+    - apt-get install -y e2fsprogs xvfb
     - cat /etc/os-release
 
 .build_stage_template: &build_stage_definition

tests/test_ext2.cc

@@ -43,6 +43,7 @@
 
 #include <iostream>
 #include <fstream>
+#include <stdlib.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <unistd.h>
@@ -203,6 +204,38 @@ TEST_F(ext2Test, Create)
 }  // namespace GParted
 
 
+// Re-execute current executable using xvfb-run so that it provides a virtual X11 display.
+void exec_using_xvfb_run(int argc, char** argv)
+{
+	// argc+2 = Space for "xvfb-run" command, existing argc strings plus NULL pointer.
+	size_t size = sizeof(char*) * (argc+2);
+	char** new_argv = (char**)malloc(size);
+	if (new_argv == NULL)
+	{
+		fprintf(stderr, "Failed to allocate %lu bytes of memory.  errno=%d,%s\n",
+			(unsigned long)size, errno, strerror(errno));
+		exit(EXIT_FAILURE);
+	}
+
+	new_argv[0] = strdup("xvfb-run");
+	if (new_argv[0] == NULL)
+	{
+		fprintf(stderr, "Failed to allocate %lu bytes of memory.  errno=%d,%s\n",
+		        (unsigned long)strlen(new_argv[0])+1, errno, strerror(errno));
+		exit(EXIT_FAILURE);
+	}
+
+	// Copy argv pointers including final NULL pointer.
+	for (unsigned int i = 0; i <= (unsigned)argc; i++)
+		new_argv[i+1] = argv[i];
+
+	execvp(new_argv[0], new_argv);
+	fprintf(stderr, "Failed to execute '%s %s ...'.  errno=%d,%s\n", new_argv[0], new_argv[1],
+		errno, strerror(errno));
+	exit(EXIT_FAILURE);
+}
+
+
 // Custom Google Test main().
 // Reference:
 // *   Google Test, Primer, Writing the main() function
@@ -210,6 +243,15 @@ TEST_F(ext2Test, Create)
 int main(int argc, char** argv)
 {
 	printf("Running main() from %s\n", __FILE__);
+
+	const char* display = getenv("DISPLAY");
+	if (display == NULL)
+	{
+		printf("DISPLAY environment variable unset.  Executing 'xvfb-run %s ...'\n", argv[0]);
+		exec_using_xvfb_run(argc, argv);
+	}
+	printf("DISPLAY=\"%s\"\n", display);
+
 	testing::InitGoogleTest(&argc, argv);
 
 	// Initialise threading in GParted to allow FileSystem interface classes to