test: downgrade to tap@^12 for continued Node 6 support

PR-URL: #1808
Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: cclauss <cclauss@me.com>

package.json

@@ -43,7 +43,7 @@
     "eslint": "^5.0.1",
     "nan": "^2.0.0",
     "require-inject": "~1.3.0",
-    "tap": "~14.2.4"
+    "tap": "~12.7.0"
   },
   "scripts": {
     "lint": "eslint bin lib test",