misc/cgo/test: make sure we run TestCrossPackageTests with appropriate options

[ianlancetaylor]

In https://golang.org/cl/163418 we started running several of the tests in misc/cgo/test via a separate invocation of go test. In cmd/dist we run the tests in misc/cgo/test with several different -linkmode options; see (*tester).cgoTest in cmd/dist/test.go. But as far as I can see those different options are not being carried through the separate invocation of go test in TestCrossPackageTests, so we are no longer testing all the cases we did before.

This issue is to make sure that we run those tests with the appropriate options.

CC @bcmills

[bcmills]

TestCrossPackageTests explicitly propagates os.Environ(), so it would probably suffice for cmd/dist to set whatever options are needed in the GOFLAGS variable before running the test.

[gopherbot]

Change https://golang.org/cl/169779 mentions this issue: cmd/dist: pass cgotest linkmode via GOFLAGS