testsuite: use $SHELL to run tests which are shell scripts

testsuite: use $SHELL to run tests which are shell scripts

* tests/Makefile.am (TEST_LOG_COMPILER): Define so that the
configure-time $SHELL is used to run the tests.
* tests/defs: Add code to re-execute by default the running
test script with configure-time $SHELL.  Updated comments.
* configure.ac: Check that `set -e' is working for $SHELL,
not for /bin/sh.
* tests/defs-static.in: Update comments.
* tests/README (Supported shells): Updated.
(Getting details from failures): Don't tell that tests are
run by /bin/sh by default.