Move the function strip_quotes to testutil.py

If one runs the testsuite with a profiling compiler, during the import
of `testlib.py`, `testlib.py` sets the global variable `gs_working`. To
do so, it executes a few statements which require the function
`strip_quotes` to be in scope. But that function only gets defined at
the very end of testlib.py.

This patch moves the definition of `strip_quotes` to testutil.py, which
is imported at the very top of testlib.py. This unbreaks the nightly
builders.

(cherry picked from commit cc07a0ba)