Make sure that we join paths correctly.

We were simply concatenating a pair of paths together but this can
cause problems if the second path is not relative. This could occur if
the OUT_DIR environment variable has been set.

Test: ./test/testrunner/testrunner.py --host -j40
Change-Id: I8b3fa817cdb114e39f9cf33a0c2e73e1a53f7e99