cmd/go: insecure 'go get' tests interfere with each other

[josharian]

Broken out from #17472.

$ go test -run=TestGoGetUpdateInsecure cmd/go
ok  	cmd/go	4.598s
$ go test -run=TestGoGetInsecureCustomDomain cmd/go
ok  	cmd/go	7.654s
$ go test cmd/go
... other test failures elided ...
--- FAIL: TestGoGetUpdateInsecure (0.01s)
	go_test.go:2753: cloning github.com/golang/example repo: exit status 128
		fatal: transport 'http' not allowed
--- FAIL: TestGoGetInsecureCustomDomain (3.21s)
	go_test.go:251: running testgo [get -d wh3rd.net/repo]
	go_test.go:270: standard error:
	go_test.go:271: package wh3rd.net/repo: cannot download, http://wh3rd.net/git uses insecure protocol
		
	go_test.go:290: testgo failed as expected: exit status 1
	go_test.go:251: running testgo [get -d -insecure wh3rd.net/repo]
	go_test.go:270: standard error:
	go_test.go:271: # cd .; git clone http://wh3rd.net/git /var/folders/pp/0010hzld5b12h_92j5th082r0000gn/T/gotest630434763/src/wh3rd.net/repo
		Cloning into '/var/folders/pp/0010hzld5b12h_92j5th082r0000gn/T/gotest630434763/src/wh3rd.net/repo'...
		fatal: transport 'http' not allowed
		package wh3rd.net/repo: exit status 128
		
	go_test.go:280: go [get -d -insecure wh3rd.net/repo] failed unexpectedly: exit status 1
... other test failures elided ...

Looks like there may be some global state that's not getting properly reset between tests.

[gopherbot]

CL https://golang.org/cl/32640 mentions this issue.