Revert "logging: Use /dev/null as test filter"

This reverts commit b6d51de6b7ab2bfeec0fd0d555837a40b6bf329b.

Change-Id: I82d29a605754013bfb1f38eacba34af1dae71526
Reviewed-on: https://code-review.googlesource.com/c/35530
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jean de Klerk <deklerk@google.com>
diff --git a/logging/logadmin/sinks_test.go b/logging/logadmin/sinks_test.go
index f88c038..aa59d61 100644
--- a/logging/logadmin/sinks_test.go
+++ b/logging/logadmin/sinks_test.go
@@ -34,8 +34,7 @@
 
 var sinkIDs = uid.NewSpace("GO-CLIENT-TEST-SINK", nil)
 
-// testFilter matches no logs to avoid exporting anything into the sinks.
-const testFilter = `textPayload="/dev/null"`
+const testFilter = ""
 
 var testSinkDestination string