New make target "testshort" runs "gotest -test.short" and is invoked
by run.bash, which is invoked by all.bash.

Use -test.short to make one package (crypto ecdsa) run much faster.
More changes to come.

Once this is in, I will update the long-running tests to use the new flag.

R=rsc
CC=golang-dev
https://golang.org/cl/4317043