tests: use `$SHELL' to run the shell scripts from `lib/'

tests: use `$SHELL' to run the shell scripts from `lib/'

This should offer greater testsuite coverage for those developers
that override CONFIG_SHELL at configure time in order to test more
shells on a single system, instead of just the default `/bin/sh'.

This change also fixes few spurious failures in tests using the
`xsi-shell' requirement, where inconsistencies could crop up if
the shell probed for XSI features (which, by default, is $SHELL)
was not the same shell later used to run the scripts using those
features (which was hard-coded to `/bin/sh').  Such failures have
already occurred in practice, for examples on Solaris systems
which had also GNU Bash installed.

* tests/ar-lib.test: Run the `ar-lib' script with `$SHELL', rather
than directly with `./ar-lib', which would make run unconditionally
with `/bin/sh'.
* tests/compile.test: Likewise, but for the `compile' script.
* tests/compile2.test: Likewise.
* tests/compile3.test: Likewise.
* tests/compile4.test: Likewise.
* tests/compile5.test: Likewise.
* tests/compile6.test: Likewise.
* tests/instsh2.test: Likewise, but for the `install' script.
* tests/instsh3.test: Likewise.
* tests/mkinst3.test: Likewise, but for the `mkinstalldirs' script.
* tests/missing.test: Likewise, but for the `missing' script.
* tests/missing2.test: Likewise.
* tests/missing3.test: Likewise.
* tests/missing5.test: Likewise.