Changes

Core tests: we need our MathML tests to use QTEST_GUILESS_MAIN() on Windows.

Otherwise, we get something like:
```
********* Start testing of MathmlTests *********
Config: Using QtTest library 5.12.9, Qt 5.12.9 (x86_64-little_endian-llp64 shared (dynamic) debug build; by MSVC 2017)
PASS   : MathmlTests::initTestCase()
QFATAL : MathmlTests::plusTests() ASSERT: "!m_baseURI.isRelative()" in file environment\qgenericstaticcontext.cpp, line 82
FAIL!  : MathmlTests::plusTests() Received a fatal error.
Unknown file(0) : failure location
Totals: 1 passed, 1 failed, 0 skipped, 0 blacklisted, 91ms
********* Finished testing of MathmlTests *********
```