x/build: shard tests over N VMs to speed all.bash

[bradfitz]

As part of the rewrite of the coordinator and building subrepos (#8646 and #9506), we also want the coordinator to use N VMs to do an all.{bash,bat,rc} instead of 1, and then shard test execution over a bunch of VMs, to speed things up.

First step is to rewrite the bottom half of all.bash into Go and ditch the three variants (bash, bat, rc).

/cc @adg

[bradfitz]

Sent https://go-review.googlesource.com/#/c/6531

[gopherbot]

CL https://golang.org/cl/10056 mentions this issue.