Linguist has some tests that don't run in CI

Linguist has some tests that live in qttools/src/linguist/tests. These appears to qtestlib-based autotests (there is a QTEST_MAIN in tst_linguist.cpp), but don't appear to be compiled or run by the CI system – the parent .pro file doesn't mention the test.

It is not clear to me how these tests relate to the tests in qttools/tests/auto/linguist. These tests should be either merged into qttools/tests/auto/linguist if they are useful or else discarded.