Update Mockttp for fixes to loopback resolution & PAC proxies

New Node versions (v20.12+) have a slightly different behaviour around
DNS lookups for IP addresses (reporting some as invalid, where they
previously just resolved to the input address). This Mockttp includes a
workaround that resolves this for most cases, allowing us to safely
update Node.

There may still be issues here with a very specific config, but likely
only for configurations using IPv6 within Docker, which is suspect is
fairly rare here. This will be resolved more completely in later
versions (probably by fixing cacheable-lookup).

Author: pimterry

package-lock.json

@@ -84,7 +84,7 @@
     "mime-types": "^2.1.27",
     "mobx": "^6.3.5",
     "mockrtc": "^0.3.1",
-    "mockttp": "^3.15.2",
+    "mockttp": "^3.15.5",
     "node-fetch": "^2.6.1",
     "node-forge": "^1.3.0",
     "node-gsettings-wrapper": "^0.5.0",