test.sh: Prefer toolbox in same directory over globally installed one

test.sh: Prefer toolbox in same directory over globally installed one

Because test.sh uses the environment's PATH, it ends up using whatever
toolbox is installed in the environment, instead of the toolbox that it
was shipped with.

This commit modifies the test script so that it overrides the path,
making tests use the toolbox they were distributed with. This also
removes unnecessary PATH modifications from the test specs.