tests: report useful system information in 'test-suite.log'

tests: report useful system information in 'test-suite.log'

It has already happened various times that a user has run the
automake testsuite, experienced a failure, read the messages
telling him "See tests/test-suite.log" and "Please report to
bug-automake@gnu.org", and done exactly that -- sending us only
the contents of `tests/test-suite.log', which are usually not
enough to start debugging the reported failure.  So we have to
ask him for more details, and usually also for the `config.log'
file generated by configure.  It's time to fix this recurring
feedback inefficiency.  We do so by creating a dummy test case
that takes care of copying the contents of `config.log', plus
other useful system information, in the final `test-suite.log'.

* tests/get-sysconf.test: New test, gathering system information
and then always terminating with a SKIP, so that its output gets
copied in `test-suite.log'.
* tests/Makefile.am (TESTS): Add it.