cwd for running dashr tests - auto and override

[alexcjohnson]

@rpkyle with any luck this will let you remove that os.chdir call in test_clientside.py.

Sets the cwd for R test apps during the subprocess.Popen call - so it won't affect any other tests. If the app was provided as a string, it tries to find the directory of the calling Python file and use that as cwd. If it was provided as a path to a file it just uses the directory of that file. Either way you can override this behavior with dashr.start_server(app, cwd='/some/path/')

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follow
  • this github #PR number updates the dash docs

[rpkyle]

@alexcjohnson The fix works for me; locally I can 🔪 os.chdir and successfully execute the test from within its own directory, as well as outside of it.

[byronz]

💃