test: rename the global service override file for coverage runs

Otherwise we break TEST-15-DROPIN, since it uses
/usr/lib/systemd/system/service.d/override.conf in some of its
sub-tests.

test/test-functions

@@ -1121,7 +1121,7 @@ install_systemd() {
     # images, since it prevents gcov to write the coverage reports (*.gcda files)
     if get_bool "$IS_BUILT_WITH_COVERAGE"; then
         mkdir -p "$initdir/etc/systemd/system/service.d/"
-        echo -e "[Service]\nProtectSystem=no\nProtectHome=no\n" >"$initdir/etc/systemd/system/service.d/override.conf"
+        echo -e "[Service]\nProtectSystem=no\nProtectHome=no\n" >"$initdir/etc/systemd/system/service.d/gcov-override.conf"
     fi
 }