debug: clicking 'debug test' twice runs two instances of the debugger simultaneously

[ramya-rao-a]

In a test file, when the debug test codelens is clicked twice, two instances of the debugger are started. Ideally, the second click should result in an error notifying that a debug session is already in progress

Steps to reproduce:

1. Set a breakpoint.
2. Click 'debug test' and wait to hit the breakpoint.
3. Step one line forward.
4. Click 'debug test' and wait to hit the breakpoint, again.

For more, including pointers on how to fix this, please see microsoft/vscode-go#1383.

[gopherbot]

Change https://golang.org/cl/240905 mentions this issue: debug: fix to prevents multiple debug sessions lanuching