runtests: clear old setenv remainders before test

Due to the layout of the singletest function there are situations where
it returns before it clears the environment variables that were
especially set for the single specific test case. That could lead to
subsequent tests getting executed with environment variables sticking
around from a previous test which could lead to badness.

This change makes sure to clear all custom variables that may be laying
around from a previous round, before running a test case.

Reported by: Kamil Dudka
Bug: http://curl.haxx.se/mail/lib-2010-08/0141.html