testsuite runtest.py cleanup (#1009)
testsuite runtest.py cleanup (#1009)

* Fix bug from PR 1001, default value for missing OSL_SOURCE_DIR env
  variable should not have included the testsuite directory itself.

* Be sure to correctly redirect all stderr to the same saved file as
  stdout. This makes it easier to have testsuite entries that test
  warning or error output.
brechtvl/OpenShadingLanguage,lgritz/OpenShadingLanguage,brechtvl/OpenShadingLanguage,aconty/OpenShadingLanguage,imageworks/OpenShadingLanguage,brechtvl/OpenShadingLanguage,aconty/OpenShadingLanguage,lgritz/OpenShadingLanguage,lgritz/OpenShadingLanguage,brechtvl/OpenShadingLanguage,aconty/OpenShadingLanguage,brechtvl/OpenShadingLanguage,imageworks/OpenShadingLanguage,imageworks/OpenShadingLanguage,imageworks/OpenShadingLanguage,aconty/OpenShadingLanguage,aconty/OpenShadingLanguage,lgritz/OpenShadingLanguage,lgritz/OpenShadingLanguage,imageworks/OpenShadingLanguage