tests: fix false failure in recent test adjustment

tests: fix false failure in recent test adjustment

* configure.ac: Comment on why we link rather than run the test,
and remove the moot __ELF__ check since we never ran it anyway,
and the new CFLAGS and LDFLAGS are a more direct test of support.
* tests/misc/wc-parallel.sh: Fix a syntax error in the previous change.
* tests/misc/md5sum-parallel.sh: Use better error checking, consistent
with that used in wc-parallel.sh.