test/config_file: correctly free context in test teardown
test/config_file: correctly free context in test teardown

Not freeing it potential leeds to using uninitialized context in follow-up
test that miss setting up context.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

rauc/rauc,rauc/rauc,jluebbe/rauc,ejoerns/rauc,jluebbe/rauc,ejoerns/rauc,rauc/rauc,ejoerns/rauc,jluebbe/rauc