tests/run-tests.sh [fix]: unset STBT_CONFIG_FILE so no user override
tests/run-tests.sh [fix]: unset STBT_CONFIG_FILE so no user override

Commit c83c42a2 "improved sandboxing" by creating an isolated environment
to run tests in. This included making a copy of test/stbt.conf and
putting into a pseudo system-default config directory. However, if the
user already has a STBT_CONFIG_FILE environment variable set, this will
take precendence over the new "system config".

This commit adds "unset STBT_CONFIG_FILE". I tested that unsetting an
environment variable does not carry through to the executing shell once
run-tests.sh has ended, so the user doesn't have to re-set it.

martynjarvis/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester