Move openTerminal() to common util file

[jmbockhorst]

Part of #2699. Moved openTerminal() into a common util file that is used by everything in test/api/. I'm not sure how to make this accessible to the addon test files.

[jmbockhorst]

I found a way to make the addons use the shared functions, but I don't love it because they have to import from ../../../out-test/api/TestUtils. I found no way to make relative paths work because mocha can't resolve them in the js files.

A better approach seems to be to use something like ts-node with mocha and test the ts files directly like this: mocha -r ts-node/register **/*.api.ts. However this caused more problems (I think due to the complex structure of the project), so I opted for the current solution.

[Tyriar]

Hmm, not really a far of referencing out but it does indeed seem to work and what I preferred doesn't (because mocha doesn't accept it as you mention). The main problem I see with it now is that the files that reference it have errors before running the first build. I guess we can go with this, if it ends up causing problems we can always go back to duplicating for addons 👍

[jmbockhorst]

Yeah, I wasn't a fan either, but I think it's better than duplicated code.