test: ignore missing coverage in TEST-02

Since c6552ad we now try to collect coverage even in situations where
it's basically impossible (like in test-mount-util where the whole / is
mounted as read-only). As dealing with this is not worth the trouble,
let's ignore the missing coverage errors thrown by gcov in such cases.

test/TEST-02-UNITTESTS/test.sh

@@ -4,6 +4,10 @@ set -e
 
 TEST_DESCRIPTION="Run unit tests under containers"
 RUN_IN_UNPRIVILEGED_CONTAINER=yes
+# Some tests make collecting coverage impossible (like test-mount-util, which
+# remounts the whole / as read-only), so let's ignore the gcov errors in such
+# case
+IGNORE_MISSING_COVERAGE=yes
 
 # embed some newlines in the kernel command line to stress our test suite
 KERNEL_APPEND="

test/test-functions

@@ -1406,8 +1406,8 @@ check_coverage_reports() {
     # usually due to the sandbox being too restrictive (e.g. ProtectSystem=yes,
     # ProtectHome=yes) or the $BUILD_DIR being inaccessible to non-root users - see
     # `setfacl` stuff in install_compiled_systemd().
-
-    if "${JOURNALCTL:?}" -q --no-pager -D "${root:?}/var/log/journal" --grep "profiling:.+?gcda:[Cc]annot open"; then
+    if ! get_bool "${IGNORE_MISSING_COVERAGE:=}" && \
+       "${JOURNALCTL:?}" -q --no-pager -D "${root:?}/var/log/journal" --grep "profiling:.+?gcda:[Cc]annot open"; then
         derror "Detected possibly missing coverage, check the journal"
         return 1
     fi