Configure Xdebug port and override XDEBUG_MODE env

[zobo]

Todo for myself.

When we Debug or Profile a test, the plugin instructs Xdebug to use xdebug_mode debug or profile. However since we allow the phpunit process to inherit the surounding environment, the XDEBUG_MODE that has preference over command line -d or .ini values can interfier with this process.
Should change so that:

• ether set env XDEBUG_MODE instead of -d or,
• leave -d and override XDEBUG_MODE with empty string

The second part is regarding which port Xdebug and php-debug extension should use. Ideally phpunit would run on a randomized port, so to not conflict with any other existing xdebug process. However since it's php-debug that is actually responsible for opening the port, a more complex 2-way communication between the two vscode extensions would be needed. An easy workaround would be to allow the user to set a specific TCP port that the two extensions should use to coordinate the DBGP session.