Added JRuby.runtim.warnings.disable_warnings_for_tests, a better way to disable these GC warnings.
MRI 1.9 tests: Fix all warnings about set_trace_func and the --debug flag.
Revert "MRI 1.9 tests: Fixed the GC.stress warning from bigdecimal/testbase.rb."

This reverts commit 5786ce752779bbc5940d0c247c7c324f03381bcb.

Conflicts:
	test/externals/ruby1.9/bigdecimal/testbase.rb
MRI 1.9 tests: reverted my change to test_coverage.rb
MRI 1.8 tests: Disabled warnings in the same general way we are doing for the other test suites.
Added test/warnings.rb and undid a lot of my changes that are no longer necessary.
Successfully ran all mri and mri19 tests without warnings.
MRI 2.0 tests: Fixed the same old warning from test_marshal.rb.
MRI 2.0 tests: Fixed another warning, but these tests have several failures so I will probably not be running them.