Allow run-tests.sh to be run from any directory.
Allow run-tests.sh to be run from any directory.

Setting `testdir` was pointless because I immediately changed directory,
rendering the relative path in testdir invalid.

This is quite important because run-tests.sh is run from the parent
directory as part of `make check`!

wmanley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester