tests: Explicitly unset DEBUGINFOD_URLS.

If DEBUGINFOD_URLS is set various tests will try to query the debuginfod
server which can stall the tests a bit. If other evironment variables
like DEBUGINFOD_PROGRESS are set it will make various tests fail because
the expected output doesn't match. Tests should PASS without needing a
debuginfod server, unless they test (and set) one themselves.

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