MdePkg/Include: Hook DebugLib _ASSERT() for unit tests

MdePkg/Include: Hook DebugLib _ASSERT() for unit tests

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2801

Update DebugLib.h _ASSERT() macro to check if unit testing
is enabled and call UnitTestDebugAssert() instead of
DebugAssert() so the an ASSERT() condition that is triggered
by a function under test can be handled by the Unit Test
Framework.

If EDKII_UNIT_TEST_FRAMEWORK_ENABLED is not defined, then
the existing DebugLib behavior is preserved.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>