Merge pull request swiftlang#21536 from compnerd/interpret

test: execute `round-trip-test` with the interpreter

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(