Allow tests project in Test Panel Tree to be sorted by subdir project

Right now unit tests in Test Panel can be only sorted "alphabetically" and "naturally", grouped by the kind of test platform (QtTest, BoostTest, Gtest).

Consider now this project structure

apps (subdir)

  - ...

libs (subdir)

  - lib1

  - lib2

  - ...

tests (subdir)

  - lib1_tests (subdir)

    - test1

    - test2

  - lib2_tests (subdir)

    - test1

    - test2

  - some_other_logical_grouping_tests

It would be tremendously useful having in Test Panel Tree an option to show the tests grouped by subdir project. In the case above I could select and run all the tests from a single group or library without selecting the manually every time.