Fix and improve CMake test finding and running

Summary:
This is a bunch of small changes that together enable running tests from
CMake with support for QtTest and viewing results in the tool view.
The changes include:

1. Enable listing test cases from tests defined with CMake and QtTest
   • Extract sources for each target from cmake server
   • Use target sources to look for QtTest declarations in them
   • Fix the search for QtTest declarations, so that it works without access to the main() function body

2. When running tests, parse the output and report results
   • Exctract output directly from the OutputModel, which is long-lived, and not from the shorter-lived OutputJob. The previous method would lose some lines of output.
   • Wait for asynchronous output signals from OutputModel, which again prevents losing some lines of test program output.

3. A new unit test that checks the cases parsed from source code, using the existing testing project unit_tests_kde.

With these changes, the test suites and individual test cases appear in the
Unit Tests tool view:

Reviewers: KDevelop, apol

Reviewed By: KDevelop, apol

Subscribers: apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D9919