Use more portable paths in the test explorer

[jennybc]

Addresses #1808 (comment)

With the R extension's log level set to "debug", you can get this info about the error:

Error: ℹ Loading vscodereporter
Error: '\U' used without hex digits in character string (<input>:1:32)
Execution halted

I was able to reproduce the problem in a windows VM and this fixes it for me.

Relevant context:

• Prior to this PR, the test-running command looked something like this:

  "C:\Program Files\R\R-4.3.2\bin\R.exe" --no-echo -e "devtools::load_all('d:/Users/jenny/source/repos/positron/extensions/positron-r/resources/testing/vscodereporter');devtools::test_active_file('d:\Users\jenny\source\repos\rematch2\tests\testthat\test-all.R', desc = 'corner cases', reporter = VSCodeReporter)"
  

  Note the paths in the snippet of R code and, specifically, the different path separators. The problem was the Windows-style path where the backslashes aren't escaped, i.e. doubled. In the error above, you can tell that the load_all() command actually succeeds, so it's the path in test_active_file() that is the problem. The solution is to use POSIX-style path separation. Here's how the test-running command looks with this PR:

  "C:\Program Files\R\R-4.3.2\bin\R.exe" --no-echo -e "devtools::load_all('d:/Users/jenny/source/repos/positron/extensions/positron-r/resources/testing/vscodereporter');devtools::test_active_file('d:/Users/jenny/source/repos/rematch2/tests/testthat/test-exec.R',reporter = VSCodeReporter)"
  

• This treatment was already in place for the path to the embedded test reporter package and now it's clear why.

  positron/extensions/positron-r/src/testing/runner-testthat.ts

  Lines 17 to 19 in 9362409

  	const testReporterPath = path
  	.join(EXTENSION_ROOT_DIR, 'resources', 'testing', 'vscodereporter')
  	.replace(/\\/g, '/');

• This approach is consistent with the advice given in the R for Windows FAQ:

  Backslashes have to be doubled in R character strings, so for example one needs "d:\\R-4.3.2\\library\\xgobi\\scripts\\xgobi.bat". You can make life easier for yourself by using forward slashes as path separators: they do work under Windows.

• FWIW child_process.spawn(..., { shell: true }) is launching a cmd.exe shell on Windows, but this turns out to be neither here nor there in terms of this problem.