Core tests: we need our MathML tests to use QTEST_GUILESS_MAIN() on W…

…indows.

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 *********
```

src/plugins/miscellaneous/Core/tests/mathmltests.cpp

@@ -284,7 +284,7 @@ void MathmlTests::trigonometricTests()
 
 //==============================================================================
 
-QTEST_APPLESS_MAIN(MathmlTests)
+QTEST_GUILESS_MAIN(MathmlTests)
 
 //==============================================================================
 // End of file