ENH: Expose original path to feedsRun script via PYFEEDS_FEEDSRUN_PATH environment variable

When pyfeeds runs a feedsRun script, it executes a copy of the script and the contents of its containing directory. This is to ensure that the feedsRun script does not inadvertently write any temporary/intermediate files to the original test directory. Some test scripts may however need access to other "in-source" resources, e.g. when executed from a checkout of the project source code. These scripts can use the PYFEEDS_FEEDSRUN_PATH environment variable to locate such resources.