sha3: move test data to testdata

Change the location of the KAT data for consistency with
other packages.

Change-Id: Ica10ad7d1730603f957fb413e28fe771b3c323bc
Reviewed-on: https://go-review.googlesource.com/3267
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

sha3/sha3_test.go

@@ -23,7 +23,7 @@ import (
 
 const (
 	testString  = "brekeccakkeccak koax koax"
-	katFilename = "keccakKats.json.deflate"
+	katFilename = "testdata/keccakKats.json.deflate"
 )
 
 // Internal-use instances of SHAKE used to test against KATs.