parallel-tests: automake error our on invalid TEST_EXTENSIONS

parallel-tests: automake error our on invalid TEST_EXTENSIONS

This change fixes automake bug#9400.

* automake.in (handle_tests): Bail out if a suffix specified in
TEST_EXTENSIONS would produce an invalid `xxx_LOG_COMPILER'
variable or an invalid suffix rule.  Before this change, automake
would have issued a confusing error messages (about invalid or
non-POSIX variables being defined), and in some situations would
have even produced a broken `Makefile.in' file.
($TEST_EXTENSION_PATTERN): New helper variable.
* doc/automake.texi (Simple Tests using parallel-tests): Document
the limitations on TEST_EXTENSIONS explicitly.
* NEWS: Update.
* tests/test-extensions.test: New test.
* tests/Makefile.am (TESTS): Update.