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.