x/tools/go/packages: TestOverlayModFileChanges broken at head

[bcmills]

I'm not sure why this isn't showing up in the builders, but TestOverlayModFileChanges is failing consistently for me on everything from go1.12.9 to gotip:

~/src/golang.org/x/tools$ git checkout master
Already on 'master'
Your branch is up to date with 'origin/master'.

~/src/golang.org/x/tools$ git status
On branch master
Your branch is up to date with 'origin/master'.

nothing to commit, working tree clean

~/src/golang.org/x/tools$ go1.13rc2 test ./go/packages
--- FAIL: TestOverlayModFileChanges (0.43s)
    packages_test.go:1119: expected module mod2

        go 1.11
        , got module mod2

        go 1.11

        require golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7 // indirect
FAIL
FAIL    golang.org/x/tools/go/packages  10.118s
FAIL

~/src/golang.org/x/tools$

CC @matloob @ianthehat

[gopherbot]

Change https://golang.org/cl/192397 mentions this issue: go/packages: set GOPACKAGESDRIVER=off for all the tests

[matloob]

For context, this was probably happening because some tests weren't setting GOPACKAGESDRIVER=off, which allowed a user's environment to affect the tests.

[matloob]

(Could you reopen if the change doesn't fix the issue?)

[bcmills]

Yep, that did the trick.