cmd/go: in 1.15: change in "go test" argument parsing [1.15 backport]

[gopherbot]

@bcmills requested issue #40763 to be considered for backport to the next 1.15 minor release.

@gopherbot, please backport to 1.15

[gopherbot]

Change https://golang.org/cl/248726 mentions this issue: [release-branch.go1.15] cmd/go/internal/test: keep looking for go command flags after ambiguous test flag

[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 76a89d6 to release-branch.go1.15.