[release-branch.go1.24] testing: allow manual timer control in testing.B.Loop

[release-branch.go1.24] testing: allow manual timer control in testing.B.Loop

Fixes #72934

Change-Id: I56610d2d11d151a8f95b6434bbedbfcd5c11c317
Reviewed-on: https://go-review.googlesource.com/c/go/+/658975
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/660555
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>