test: move the debugger tests back to parallel

Run the debugger with `--port=common.PORT` to avoid the use of the same
port.

PR-URL: https://github.com/nodejs/node/pull/6246
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-debugger-repeat-last.js

@@ -12,6 +12,7 @@ const fixture = path.join(
 
 const args = [
   'debug',
+  `--port=${common.PORT}`,
   fixture
 ];
 

test/parallel/test-debugger-util-regression.js

@@ -12,6 +12,7 @@ const fixture = path.join(
 
 const args = [
   'debug',
+  `--port=${common.PORT}`,
   fixture
 ];