tests: make tests behave for the AR=lib case

tests: make tests behave for the AR=lib case

These below tests were left mostly as-is when AM_PROG_AR was
introduced (with libobj16b being the exception, that test was
forgotten or has regressed since), in a misguided attempt to
not make them diverge from the original problem reports.

Modernize them so that they do not FAIL when $AR is Microsoft
'lib'.

* tests/libobj16b.test: Add AM_PROG_AR to configure.ac and remove
the no-longer-needed -Wno-extra-portability option when invoking
automake.
* tests/pr300-lib.test: Likewise.
* tests/pr300-ltlib.test: Likewise.
* tests/pr307.test: Likewise.
* tests/pr401.test: Likewise.  Also fetch the 'ar-lib' script.
* tests/pr401b.test: Likewise.
* tests/pr401c.test: Likewise.