tap/awk: "Bail out!" recognized also after leading whitespace

tap/awk: "Bail out!" recognized also after leading whitespace

Newer versions of TAP::Harness (e.g., 3.23 on Perl 5.14.1)
recognize a "Bail out!" directive also when it is prepended by
leading whitespace; this was not the case for older TAP:Harness
versions, (e.g., for version 3.17 on Perl 5.12.4), and for our
TAP driver implemented in awk.

* lib/tap-driver.sh: Handle the "Bail out!" directive also when
it is preceded by leading whitespace.
* tests/tap-spurious.test: Remove the tests checking that a
"Bail out!" string coming right after leading whitespace does
not trigger a bailout action.
* tests/tap-bailout-leading-space.test: New test.
* tests/list-of-tests.mk: Add it.

Problem reported by Jim Meyering in automake bug#10374.