test.bash: purge unused directories based on timeout
test.bash: purge unused directories based on timeout

Rather than manually keeping track of which directories may be used
by other branches, use a time-based approach to determine whether
to purge a sub-directory or not.

Change-Id: I648dcca075e351e4e5624c48e73792782fc349ca
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/167042
Reviewed-by: Herbie Ong <86f376bf42b6daa37c768c730acd318ab6855de4@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go