fuse/runtests: added --tests, renamed --suites

fuse/runtests: added --tests, renamed --suites

changed the --tests option to be --suites, as it takes a prefix, e.g. 'read'
'write' (or 'all', the default) and runs those suites which are applicable to
each implementation being tested.

added a --tests option, which takes a list of tests, e.g. 'read_file_contents'
'write_overlapping_large_writes' and runs all tests specified without regard
to whether the implementation(s) under test are declared to support them.

this is basically to allow a specific test or two to be run, saving time
during development and debugging by not running the entire suite