testing: stub out testing.B.Loop

This gets the path package tests to pass, so we can move ahead with Go
1.25. It should be implemented in the future at some point (that, or
we'll use the upstream testing package instead).

src/testing/benchmark.go

@@ -500,6 +500,10 @@ func (b *B) RunParallel(body func(*PB)) {
 	return
 }
 
+func (b *B) Loop() bool {
+	panic("unimplemented: testing.B.Loop")
+}
+
 // Benchmark benchmarks a single function. It is useful for creating
 // custom benchmarks that do not use the "go test" command.
 //