Prevent many TC triggered by bad tag expressions

Typical example: Use multiple perftest lines in a comment.
Atypical example: Use perftest as an expression (without any ANDs).

Fix: Die if "perftest" is found in the tag expression (before prefixing).

Change-Id: I0230541b30557ce7a4e9d57c8303cd432f86747d
Signed-off-by: Vratko Polak <vrpolak@cisco.com>