cmd/test2json: tests that panic are marked as passing [1.15 backport]

[gopherbot]

@bcmills requested issue #40132 to be considered for backport to the next 1.15 minor release.

@gopherbot, please backport to Go 1.15. This seems like a significant regression.

[gopherbot]

Change https://golang.org/cl/248725 mentions this issue: [release-branch.go1.15] cmd/test2json: do not emit a final Action if the result is not known

[dmitshur]

We've considered this in a release meeting. Approving as it is a serious issue without a workaround. Only 1.15 needs this backport.

[gopherbot]

Closed by merging 09b2897 to release-branch.go1.15.