tests: avoid spurious failures under "make distcheck"

tests: avoid spurious failures under "make distcheck"

* tests/tap-setup.sh: Pass also '-f' option to 'cp' invocations,
so that files that have already been copied in without write
permissions will still be overwritten where needed, instead of
causing a spurious testsuite failure.  Issue seen when the
testsuite is run under "distcheck".
* tests/defs (get_shell_script): Remove target file before
overwriting it, for the same reason.  Rework code flow a bit
since we are at it.