tests: Only export DEBUGINFOD_URLS when necessary

tests: Only export DEBUGINFOD_URLS when necessary

A couple of test set DEBUGINFOD_URLS before starting a debuginfd
server causing the server to query itself or a nonexisting debuginfod
server as delegate. In most cases it should be set after, except for
the testcase that explicitly checks for errors when using an invalid
URL.

Signed-off-by: Mark Wielaard <mark@klomp.org>