core: revert "manager: do not set up signals in test mode"

This reverts commit 5aa1054521.

Fixes test-execute
$ sudo make check TESTS=test-execute
...
$ cat test-execute.log
+ test /tmp/test-exec_workingdirectory = /tmp/test-exec_workingdirectory
Test timeout when testing exec-workingdirectory.service
exec-workingdirectory.service
	UMask: 0022
	WorkingDirectory: /tmp/test-exec_workingdirectory
	RootDirectory: /
	NonBlocking: no
	PrivateTmp: no
	PrivateNetwork: no
	PrivateDevices: no
	ProtectHome: no
	ProtectSystem: no
	IgnoreSIGPIPE: yes
	RuntimeDirectoryMode: 0755
	StandardInput: null
	StandardOutput: inherit
	StandardError: inherit
FAIL test-execute (exit status: 1)

src/core/manager.c

@@ -410,9 +410,6 @@ static int manager_setup_signals(Manager *m) {
 
         assert(m);
 
-        if (m->test_run)
-                return 0;
-
         assert_se(sigaction(SIGCHLD, &sa, NULL) == 0);
 
         /* We make liberal use of realtime signals here. On