Add a test_options for deno test

[pipelight]

Make sure these boxes are checked before submitting your pull request:

• Add fixtures and spec when implementing or updating a test runner
• Update the README accordingly
• Update the Vim documentation in doc/test.txt

It seems like it hasn't been implemented for every runner.

S/O to maintainers great tool!

[codeinabox]

Thank you for your PR. I've looked into this, and you are able to useg:test#javascript#denotest#options to set options, which I've validated with this test #798. Does that achieve what you need?

[codeinabox]

I've realised the existing documentation was a bit confusing, as it didn't make clear that test_options was specific to cargotest. I've got a PR #799 to update it, could you let me know if this clears things up?

[pipelight]

Thank you very much for figuring this out.
My bad for submitting such a low quality PR, I was to much in a hurry to browse the documentation.
Great tool! Thanks again for maintaining it!