Fix --blacklist when --jobs > 1 in run_tests.py.

The options parsing logic replaces the options.blacklist initial value,
a list of filenames, with a dict of test names to filenames.  When
runchildren rebuilds the command line to launch child processes the dict
is flattened and provided as a malformed argument when a filename is
expected.

Fix is to remove the blacklist option from child invocations since the
filtering is handled in the parent anyway.

tests/run-tests.py

@@ -657,6 +657,7 b' def runchildren(options, tests):'
 
     optcopy = dict(options.__dict__)
     optcopy['jobs'] = 1
+    del optcopy['blacklist']
     if optcopy['with_hg'] is None:
         optcopy['with_hg'] = os.path.join(BINDIR, "hg")
     optcopy.pop('anycoverage', None)