Make tests that wait for concurrent work wait on the event or a condition with a long deadline (issue 1814)

A test that slept a fixed time and then asserted what a goroutine, process or
watcher did failed under the checker's load. Each now waits for its positive
evidence (a started baseline, a call counted, a state reached) within 30 s, and
the concurrency-limit tests meet at a barrier instead of overlapping by sleep.

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

modules/restic/cmd/restic-backups/data_test.go

@@ -140,7 +140,7 @@ func TestARetiredItemIsDeletedOnlyAfterALastRestorePoint(t *testing.T) {
 
 	wait := func(path string) Deletion {
 		t.Helper()
-		for i := 0; i < 200; i++ {
+		for i := 0; i < 6000; i++ {
 			if d, err := b.DeletedOutcome(path); err == nil && d.Done {
 				return d
 			}