test/py: Quote consistency

When converting test/py from " to ', I missed a few places (or added a
few inconsistencies later). Fix these.

Note that only quotes in code are converted; double-quotes in comments
and HTML are left as-is, since English and HTML use " not '.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>