CI: Optimize use of "make realclean" in looped test cases and for absent git

[jimklimov]

After #1159 we can thoroughly clean the workspace and check that it happened.

1. The check relies on a git checkout however, to compare that there are no positive or negative changes, so if there is no .git in the workspace, this can be skipped.

• Also need to investigate why it is sometimes missing in CI builds: at some point it was not "stashed" as a "hidden" FS pattern, but then it was fixed... did that break again?

2. For looped tests (e.g. default-allerrors checking several ifdef-ed code paths) there is noticeable overhead for re-running autogen.sh (especially in QEMU test agents). It may be beneficial to make distclean there, and conjure a target that checks that the only new files present in the workspace (and git-ignored normally) are ones defined by REALCLEAN_FILES macros in Makefile.am files, recursively.

[jimklimov]

Addressed by part of #1170