WIP: Lua unit tests

[rolandlo]

This is a proof of concept for adding unit tests for the Lua plugin API.

• app.openFile
• app.getDocumentStructure, app.setCurrentPage, app.setCurrentLayer
• app.setSidebarPageNo and app.getSidebarPageNo
• app.uiAction({action = "ACTION_NEW_PAGE_AFTER"}) (currently segfaults)

Currently gtk_widget_destroy is removed from the GladeGui destructor, since the test segfaults otherwise (this was found by attaching gdb to test/test-units and looking at the backtrace when it segfaulted).

Configure with the flag -DENABLE_GTEST=on, build with

cmake --build . --target test-units

Run the tests via

cmake --build . --target test

If a (Lua) test fails, rerun with

ctest --stop-on-failure --rerun-failed --output-on-failure

Then you get useful information, like e.g. when asserting a wrong page number (3 instead of 2) via
assert_equal(app.getSidebarPageNo(), 3) we get:

-- Starting suite "main", 2 test(s)
  F.
---- Testing finished in 19.14 ms, with 18 assertion(s) ----
  1 passed, 1 failed, 0 error(s), 0 skipped.
FAIL: main.test_sidebarPage (0.06ms): Expected 2 +/- 0, got 3 (60)


0% tests passed, 1 tests failed out of 1

Total Test time (real) =   0.21 sec

The following tests FAILED:
         22 - LuaTest.testPage (Failed)
Errors while running CTest