Add support for continuous mode in `run` and `run in terminal` test profiles

[vinistock]

VS Code supports running tests continuously upon file modifications by marking the profile with support for "continuous" runs. This doesn't apply to the debugging profile, but it would apply to the regular run profile or the run in terminal (#3312).

Implement continuous running for these profiles.

[andyw8]

Interesting. https://code.visualstudio.com/updates/v1_78#_testing

This sounds like it would partly replace guard-minitest (etc), but only in one direction (i.e. it wouldn't re-run the test when the implementation changes).