Fix error with exporting PYTHONPATH variable

If PYTHONPATH variable is already set while
defining and exporting it in 'run_test.sh' script
an error is generated. Symbol ":" should be used
as delimeter instead of space and expression
should be quoted to avoid this.

Change-Id: Iccd36e6857069abb69b2d31abf5f8a740016d887
Closes-bug: #1325979
Merge "Fix error with exporting PYTHONPATH variable"