test: ensure all failing tests exit nonzero.

Previously merely printing an error would cause the golden
file comparison (in 'bash run') to fail, but that is no longer
the case with the new run.go driver.

R=iant
CC=golang-dev
https://golang.org/cl/7310087
diff --git a/test/string_lit.go b/test/string_lit.go
index 457faaa..fea6f55 100644
--- a/test/string_lit.go
+++ b/test/string_lit.go
@@ -33,6 +33,7 @@
 				print("\ta[", i, "] = ", ac, "; b[", i, "] =", bc, "\n")
 			}
 		}
+		panic("string_lit")
 	}
 }
 
@@ -110,7 +111,7 @@
 	r = -1
 	s = string(r)
 	assert(s, "\xef\xbf\xbd", "negative rune")
-	
+
 	// the large rune tests again, this time using constants instead of a variable.
 	// these conversions will be done at compile time.
 	s = string(0x10ffff) // largest rune value