Update Mockttp for proxy additional-CA support

Author: pimterry

package-lock.json

@@ -30,7 +30,9 @@
     "test:integration:release": "cross-env TEST_BUILT_TARBALL=1 npm run test:integration"
   },
   "mocha": {
-    "node-option": ["import=tsx"]
+    "node-option": [
+      "import=tsx"
+    ]
   },
   "repository": "httptoolkit/httptoolkit-server",
   "homepage": "https://github.com/httptoolkit/httptoolkit-server",
@@ -81,7 +83,7 @@
     "mime-types": "^2.1.27",
     "mobx": "^6.3.5",
     "mockrtc": "^0.3.1",
-    "mockttp": "^3.13.0",
+    "mockttp": "^3.15.0",
     "node-fetch": "^2.6.1",
     "node-forge": "^1.3.0",
     "node-gsettings-wrapper": "^0.5.0",