tests: avoid shared lib tests on unsupported platforms

tests: avoid shared lib tests on unsupported platforms

* init.cfg (require_gcc_shared_): A new function to check
that we can build shared libraries in the particular manner
we use in our tests.
* tests/cp/nfs-removal-race.sh: Use require_gcc_shared_.
Then fail rather than skip, if the actual shared lib build fails.
* tests/df/no-mtab-status.sh: Likewise.
* tests/df/skip-duplicates.sh: Likewise.
* tests/ls/getxattr-speedup.sh: Likewise.
Reported in http://bugs.gnu.org/14024