backend: Split RetryBackend into own package

The RetryBackend tests depend on the mock backend. When the Backend
interface is eventually split from the restic package, this will lead to
a dependency cycle between backend and backend/mock. Thus split the
RetryBackend into a separate package to avoid this problem.

internal/backend/retry/testing.go

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