Add environment variable indicating debug mode

[JustinGrote]

Fixes #239

[JustinGrote]

@hbenl hello, any appetite to merge this or should I just fork?

[Spown]

Too late to the party and the author is semi-dead anyway. but there is "native", less invasive way to tell whether the dubugger is running, that does not violate the purity of testing env. Simply change the mochaExplorer.debuggerPort (preferrably workspace-scoped) to anything other than 9229 and check process.debugPort!==9229 in the code. The undebugged run always run on 9229

[JustinGrote]

I've since started using https://github.com/microsoft/vscode-test-cli and the associated testing extension which works great.