Add support for test_active_file

[ilyaZar]

Problem
A convenient way for continuous testing (in RStudio) is to only run devtools::test_active_file() to test only the active test-file.

This is especially beneficial, if the project is large and a blunt testthat::test() takes too much time. Currently, devtools::test_active_file() gives

r$> devtools::test_active_file()
Error in rstudioapi::executeCommand("activateConsole", quiet = TRUE) : 
  This {rstudioapi} function is not currently implemented for VSCode.

Solution
I assume you folks do not have much time, but I am happy to work on this, if:

• it is possible with moderate difficulty
• give me some hints on where to start e.g. other similar commands already ported from the {rstudioapi} so I can take their implementation as a blueprint

The project here is new to me, but if you give me some guidance on where to start a possible PR I'd happy to dive in!

Cheers and thanks for this extension, it's a gem !

[ThomasSoeiro]

@eitsupi @Fred-Wu @grantmcdermott Can we close in favour of #59? (If so, I will add a comment in #59 to specifically mention devtools::test_active_file().)