push tests down into new org.nlogo.headless.lang.misc package
make TestCompileBenchmarks use FixtureSuite

as part of an effort to standardize on Fixture for
HeadlessWorkspace-based testing
add comments to TestErrorMessages
minor code improvement in headless.lang.misc
split TestDuplicateNames off of TestErrorMessages
code improvement in TestFilePrimsUTF8 (use Fixture more)
rename TestParallelModels -> ParallelSuite
code improvement in TestStackTraces