Allow testsuite to run configure with alternate shells.

Allow testsuite to run configure with alternate shells.

For portability testing of autoconf macros, it is useful to be able
to have the tests run configure with different shells.  Add a global
knob to do this: just run make check with TEST_SHELL=whatever and
the testsuite will set CONFIG_SHELL in testcases appropriately.

Or configure the whole package with CONFIG_SHELL and that will get
passed down too.

However, avoid running the testsuite itself by default with such a
shell because that's not actually what we're trying to test.