go/parser: initialize file set even in only some tests are run
go/parser: initialize file set even in only some tests are run

Without this fix, some tests crashed (e.g. go test -run Invalid).

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/133580043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go