tests: allow setting environment in extra file

It can be handy to set apt options for the testcases which shouldn't be
accidentally committed like external planner testing or workarounds for
local setups.

Gbp-Dch: Ignore

test/integration/framework

@@ -433,6 +433,10 @@ EOF
 	# most tests just need one signed Release file, not both
 	export APT_DONT_SIGN='Release.gpg'
 
+	if [ -r "${TESTDIRECTORY}/extra-environment" ]; then
+		. "${TESTDIRECTORY}/extra-environment"
+	fi
+
 	msgdone "info"
 }