test: reenable syntax tests

These were fixed a little while ago, but overlooked when reenabling
disabled tests.

Update #9968.

Change-Id: I301ef587e580c517a170ad08ff897118b58cedec
Reviewed-on: https://go-review.googlesource.com/9347
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

test/run.go

@@ -51,8 +51,7 @@ var (
 
 	// dirs are the directories to look for *.go files in.
 	// TODO(bradfitz): just use all directories?
-	// TODO(rsc): Put syntax back. See issue 9968.
-	dirs = []string{".", "ken", "chan", "interface", "dwarf", "fixedbugs", "bugs"}
+	dirs = []string{".", "ken", "chan", "interface", "syntax", "dwarf", "fixedbugs", "bugs"}
 
 	// ratec controls the max number of tests running at a time.
 	ratec chan bool