tests: shell running test scripts is now named AM_TEST_RUNNER_SHELL

tests: shell running test scripts is now named AM_TEST_RUNNER_SHELL

This is just a preparatory refactoring for future changes.

* configure.ac (AM_TEST_RUNNER_SHELL): New variable, defined
to $SHELL', and AC_SUBST'd.
* Makefile.am (LOG_COMPILER): Redefine from $(SHELL) to
$(AM_TEST_RUNNER_SHELL).
* defs: Adjust to use $AM_TEST_RUNNER_SHELL instead
of $CONFIG_SHELL and/or $SHELL, where appropriate.  Minor
opportunistic changes.
* t/self-check-explicit-skips.sh: Likewise.
* t/self-check-sanity.sh: Likewise.
* t/self-check-tap.sh: Likewise.
* t/self-check-cleanup.tap: Likewise.
* t/self-check-dir.tap: Likewise.
* t/self-check-env-sanitize.tap: Likewise.
* t/self-check-exit.tap: Likewise.
* t/self-check-me.tap: Likewise.
* t/self-check-reexec.tap: Likewise.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>