BaseTools/Tests: Always add BaseTools source to import path

BaseTools/Tests: Always add BaseTools source to import path

This allows unit tests to easily include BaseTools python
modules. This is very useful for writing unit tests.

Actually, previously, we would do this when RunTests.py was executed,
so unit tests could easily import BaseTools modules, so long as they
were executed via RunTests.

This change allows running the unit test files individually which can
be faster for developing the new unit test cases.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Yingke Liu <yingke.d.liu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17691 6f19259b-4bc3-4df7-8a09-765794883524