test: execute round-trip-test with the interpreter

This is a python script that is run by tests.  However, they do not
explicitly use the interpreter.  This may result in the wrong python
interpreter being used on some systems.  On Windows this will fail to
execute since the shebang is not honoured.

Author: compnerd

test/lit.cfg

@@ -1282,7 +1282,9 @@ config.substitutions.append(
 config.substitutions.append(('%utils', config.swift_utils))
 config.substitutions.append(('%line-directive', '%r %s' % (sys.executable, config.line_directive)))
 config.substitutions.append(('%gyb', '%r %s' % (sys.executable, config.gyb)))
-config.substitutions.append(('%round-trip-syntax-test', config.round_trip_syntax_test))
+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(('%scale-test',
                              '{} --swiftc-binary={} --tmpdir=%t'.format(