lib: Address https://v.io/i/1220

- reverts https://v.io/c/20483
- changes v23test to not share a bin dir across test main's
(bin dir is still shared across tests within a given test
main, but that's safe since "go test" will not run such
tests concurrently by default)
- changes v23test.StartRootMountTable and
v23test.StartSyncbase to use gosh.Shell.FuncCmd
(refactoring syncbased and mounttabled as needed to make
this possible)

A side effect of this change is that tests should run much
faster, because for the common case of tests that previously
used StartRootMountTable and/or StartSyncbase, but didn't
directly call BuildGoPkg, we'll no longer build Go packages
during test execution.

MultiPart: 2/5

Change-Id: Iba81f1db56ca21f2d887e26496d7a87ad288b076
3 files changed