speed-up integration tests by reducing the RetryBackend timeout

On my machine this decreases the runtime for `./cmd/restic` from 9.5s to
6.5s.

internal/backend/testing.go

@@ -0,0 +1,8 @@
+package backend
+
+import "testing"
+
+// TestFastRetries reduces the initial retry delay to 1 millisecond
+func TestFastRetries(t testing.TB) {
+	fastRetries = true
+}