This follows the naming convention for global headers and causes
qttestglobal.h to be added to the top of the generated QtTest header. It
is necessary to have it there when other headers depend on features
defined in configure.json.

[ChangeLog][QtTest] The qtest_global.h header is now deprecated. Include
qttestglobal.h instead.

Change-Id: Iaed639d4e13dd99cee6387fce9d15d6e55b0b1e8
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>