[Tests][Backtracing] Fix JSON test failure on Linux.

It's possible that the last frame in the test is genuinely the
last one we see in the backtrace, in which case there won't be
a `,` after the `}`.

rdar://121430255

Author: al45tair

test/Backtracing/JSON.swift

@@ -201,9 +201,9 @@ struct Crash {
 // CHECK-NEXT:           "line": 0,
 // CHECK-NEXT:           "column": 0
 // CHECK-NEXT:         }
-// CHECK-NEXT:       },
+// CHECK-NEXT:       }
 
-// More frames here, but they're system specific
+// Possibly more frames here, but they're system specific
 
 // CHECK:          ]
 // CHECK:        }