tests: make test runner a script, not a shell function

tests: make test runner a script, not a shell function

This change implements a more correct and idiomatic use of the
features of the Automake-provided 'parallel-tests' harness.
Moreover, this change is required in order for the testsuite to
continue to work with the new testsuite harness that is planned
to be introduced in Automake 1.12 (which, as of the writing date,
is still under development and in alpha state).

* tests/shell-or-perl: New auxiliary script.
* tests/Makefile.am (EXTRA_DIST): Distribute it.
* tests/check.mk (TESTS_ENVIRONMENT): Remove definition of the
`shell_or_perl_' shell function, whose code has been moved in
the new script above (with a few improvements and extensions).
Do not use it to run the test scripts.
(LOG_COMPILER): New, properly invoking `shell-or-perl'.