tests: few improvements to some `ar-lib' related tests

tests: few improvements to some `ar-lib' related tests

* tests/ar-lib5a.test (Makefile.am): Also check that the target
library has truly been created.
(ar-lib): Use the real `ar-lib' script (mildly patched) rather
than a dummy one, to ensure better "real-life coverage".  Fix
botched shebang line.
* tests/ar-lib5b.test: Extend the PATH variable to make the dummy
`lib' script accessible, instead of explicitly calling it by its
relative/absolute path.
(ar-lib): Fix botched shebang line.
(bin/lib): Likewise.  Also, add explicative comments, and make
slightly stricter.