cmd/dist: add test that Go can bootstrap itself with just-built tree

[bradfitz]

CL 166985 broke the build, but in a way that the build system didn't detect: it produced a build that couldn't be used to as a GOROOT_BOOTSTRAP directory.

We should add a test (perhaps off by default, except on at least 1 builders, including trybots) that just runs make.bash again using the just-built tree as its GOROOT_BOOTSTRAP.

Because we shard tests and slow tests are scheduled first, it shouldn't affect the overall trybot time.

/cc @mdempsky @bcmills @dmitshur

[gopherbot]

Change https://golang.org/cl/167478 mentions this issue: cmd/dist: add a test in misc/reboot to verify that the toolchain can self-bootstrap