cmd/coordinator: note that cmd/compile/internal/gc tests are slow
cmd/coordinator: note that cmd/compile/internal/gc tests are slow

Until we can use historical data, keep this roughly-accurate handmade
table up to date. This helps with the coordinator's critical path
scheduling of tests over buildlets. (I noticed this stuck at the end
of a build, rather than the beginning)

Change-Id: I6e64b138b1f5b3163157d93c59ba19c25c959730
Reviewed-on: https://go-review.googlesource.com/21966
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/build,golang/build,golang/build,golang/build