[usdMaya] [tests] cmake should use $MAYA_BASE_DIR not $MAYA_LOCATION

[pmolodo]

...as this is the only cmake-variable guaranteed to be set after FindMaya runs

Description of Change(s)

Makes the CMakeLists.txt use $MAYA_BASE_DIR, since $MAYA_LOCATION may not be defined.

Included Commit(s)

• 11ed4f3

Fixes Issue(s)

• Allows tests to work properly in situations where cmake was not called with an explicit "-D MAYA_LOCATION=/path/to/maya", but instead relies on setting $MAYA_LOCATION as shell environment variable (where it is then converted to the MAYA_BASE_DIR cmake-variable)