run-tests: outputdir also has to be changed if $TESTDIR is not $PWD
Following a18eef03d879, running run-tests.py from outside tests/ would lead to
the creation of .testtimes and test-*.t.err in $PWD instead of $TESTDIR. This
patch fixes that and updates the relevant test.
................
----------------------------------------------------------------------
Ran 16 tests in 0.000s
OK