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.