[cxxmodules] Disable tricky to handle test.

There are two problems with this test and cxxmodules. First, it picks up a
header files from $ROOTSYS/include and builds a library. Naturally, we should
build pcm files, however, building module MathCore requires specific -I to
build the needed VecCore and Vc components. This test does not have access
to the include relevant include paths. Secondly, if we want to reuse the
modules from ROOT which makes most sense we can't because we get a hard
error such as: fatal error: malformed or corrupted AST file:
'SourceLocation remap refers to unknown module, cannot find include/pcms/1WYSNQV9VBZK7/stl-2OZGQN92C38MI.pcm

We can fix the first point by moving out all VecCore-related headers
such as Math/Types.h and all of its includers in a separate module. Thus,
the current test will not require the VecCore (as in the textual case).
Alternatively, we can trace the origin of the fatal error and try to remap
the source locations.