test: execute rth with the interpreter

Not all targets support the shebang for the interpreter.  In particular,
Windows does not support this.  Add an explicit interpreter that is used
to invoke the tool.

Author: compnerd

test/lit.cfg

@@ -1488,7 +1488,7 @@ config.substitutions.append(('%gyb', '%r %s' % (sys.executable, config.gyb)))
 config.substitutions.append(('%round-trip-syntax-test',
                              '%r %s' % (sys.executable,
                                         config.round_trip_syntax_test)))
-config.substitutions.append(('%rth', config.rth))
+config.substitutions.append(('%rth', '%r %s' % (sys.executable, config.rth)))
 config.substitutions.append(('%scale-test',
                              '{} --swiftc-binary={} --tmpdir=%t'.format(
                                  config.scale_test, config.swiftc)))