testing: Helper line number has changed in 1.16 [1.16 backport]

[gopherbot]

@ianlancetaylor requested issue #44887 to be considered for backport to the next 1.16 minor release.

@gopherbot Please open backport to 1.16

In some cases t.Helper seems to be ineffective. It's not critical but it's definitely annoying. We should fix it.

[gopherbot]

Change https://golang.org/cl/303189 mentions this issue: [release-branch.go1.16] testing: update helperNames just before checking it

[toothrot]

Approved. There's no workaround and this regression makes t.Helper not very useful.

[gopherbot]

Closed by merging 902d16e to release-branch.go1.16.