goose: read tests from testdata

Since tests are all written in terms of Go source, it makes tests easier
to write.  They still need to be arranged in a GOPATH for go build, but
tests that just call Generate can operate in-place because I've faked
the filesystem.

Reviewed-by: Tuo Shan <shantuo@google.com>
Reviewed-by: Herbie Ong <herbie@google.com>

internal/goose/testdata/MissingUse/foo/foo_goose.go

@@ -0,0 +1,5 @@
+//+build gooseinject
+
+package main
+
+func injectedMessage() string