Run skipif files as scripts

This patch adds the capability to run <test>.skipif, <subdir>.skipif and SKIPIF files as scripts, augmenting the current behavior where these are piped through the util/test/testEnv script.
The scripts that use testEnv now check to see if the skipif file is executable.  If so, the file is run as a script and its output is used to determine the skipping behavior.  If not, then the file is used as input to testEnv, and the output of testEnv is used to determine whether the test is run (just as before).
The output from the script can be either "1" or "True" to skip the associated test or directory.
I think this should check the executable bit like everything else, for consistency.