tests: Allow expansion of mocked stat symbols

tests: Allow expansion of mocked stat symbols

When libc uses a define to rewrite stat64 to stat our mocks do not work if they
are chained because the symbol that we are looking up is being stringified and
therefore preventing the stat64->stat expansion per C-preprocessor rules.  One
stringification macro is just enough to make it work.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>