Fix logic in base directory check for macOS

[martyngigg]

Description of work.

Fixes if to return early if tested file path exists in ConfigService when searching for Mantid.properties.

The OSX system tests are currently failing on master because of the error: http://builds.mantidproject.org/job/master_systemtests-osx/

To test:

• Build a package on macOS and install it
• Run /Applications/MantidPlot.app/Contents/MacOS/mantidpython --classic
• import mantid should succeed now

No issue

Does not need to be in the release notes. A regression introduced in #18680.

---

Reviewer

Please comment on the following (full description):

Code Review

• Is the code of an acceptable quality?
• Does the code conform to the coding standards? Is it well structured with small focussed classes/methods/functions?
• Are there unit/system tests in place? Are the unit tests small and test the a class in isolation?
• If there are changes in the release notes then do they describe the changes appropriately?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?

• How do the changes handle unexpected situations, e.g. bad input?

• Has the relevant documentation been added/updated?

• Is user-facing documentation written in a user-friendly manner?

• Has developer documentation been updated if required?

• Does everything look good? Comment with the ship it emoji but don't merge. A member of @mantidproject/gatekeepers will take care of it.

[samueljackson92]

Checked out the code and built the package. Followed the test instructions and I can now import Mantid successfully.