Fix 887 and improve code coverage of R/config.R from 28.21% to 100%

[ilyaZar]

Fix #887

Problem:
Whenever golem::document_and_reload() calls get_golem_wd() to retrieve values from a yml-config, and the user changed the location of that yml before that call, golem::document_and_reload() is not aware of that location change and fails. Specifically, guess_where_config() cannot find the location when it is not of standard type (e.g. "inst/golem-config.yml").

Fix:
This PR solves this by adding a helper to guess_where_config() which finds the user config-yaml by reading its new location from user changes in "R/app_config.R".

[ALanguillaume]

Thanks a lot @ilyaZar !

I will take the time to review your PR asap. In the mean time I quickly eye-balled on the code and made a few comments.

[ilyaZar]

Done:

Just refactored the logic for the whole file because it got complicated and messy. Here is a summary of all changes of this PR so far:

• 31213b0: add a helper named try_user_config_location() inside guess_where_config() which finds a user supplied config-yaml by reading its new path location from changes in the "R/app_config.R" file
• 83a96a7: fix a minor style issue - use && instead of & with logical scalars
• 230b73d: add tests for this behavior checking that the config-yaml can be:
  1. renamed and
  2. moved to another location
• 8e98808: improve readability and logic of try_user_config_location(); also add support for multi-line statements of app_sys() inside R/app_config.R; update tests to check the new behaviour improving code coverage of R/config.R to 63.16%
• 90ee1ca: sub-function should output same object (fs-path type) to top level function to make comparison-checks easier; and fix a typo
• a941ec1: rework guess_where_config() -> since the user can now set paths to his/her golem-configs this must be properly handled
• a941ec1: add proper tests to check the previous behavior and improve code coverage for R/config.R to 68.87%
• 095bfc5: add more tests to cover "exotic" corner cases when trying to find the golem-config; moves code coverage of the file to 70.75%
• 3917328: remove redundant if-statement which is roughly a duplicate of the one before; worst case is that guess_where_config() fails slightly faster which is probably good as we can then handle the failure as another corner case explicitly
• f3e6cb2: encapsulate yesno() into separate function call to improve function logic and make guess_where_config() testable in interactive mode via the package {mockery}
• caf682a: based on the previous, add the mocking tests to obtain a coverage of 100% for R/config.R

• 6529474: document the implemented features

To-Do:

• inform the user with a print statement which config is used, or add a helper that prints the current location of the config in use?

[ilyaZar]

@VincentGuyader do you think above CI failures relate to #1072 or #1070 ? my forked branch passed the tests on my local machine hence the question.

if you point me somewhere I'll try to fix the PR myself to make it pass the CI. thanks!