There was still the word 'Hashmap' in gc_test.go, so I renamed it to just 'Map'

Previous renaming commit: https://golang.org/cl/90336

Change-Id: I5b0e5c2229d1c30937c7216247f4533effb81ce7
Reviewed-on: https://go-review.googlesource.com/96675Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>