maint: tests: refactor gcc commands for building shared lib

maint: tests: refactor gcc commands for building shared lib

* init.cfg (gcc_shared_): A new function refactored from tests.
(require_gcc_shared_): Adjust to call gcc_shared_() to build the
test library, and remove that library before the function returns.
* tests/cp/nfs-removal-race.sh: Call the new gcc_shared_().
* tests/df/no-mtab-status.sh: Likewise.
* tests/df/skip-duplicates.sh: Likewise.
* tests/ls/getxattr-speedup.sh: Likewise.
* tests/rm/r-root.sh: Likewise.