Auto merge of #18984 - dsandeephegde:master, r=jdm

Initial steps of Mutation testing

<!-- Please describe your changes on the following line: -->
- Added one strategy of mutation which is replacing occurrences && to ||.
- Added test mapping framework for running mutation tests corresponding to a mutant.
- Added one test_mapping.json to map source file in a folder to WPT test.
- Added README mentioning about Mutation testing.
- Added CI script to invoke mutation test.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes #18529 (github issue number if applicable).
- [x] These changes do not require tests because it is a python script to run mutation test and does not change any behavior.

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etc/ci/buildbot_steps.yml

@@ -68,6 +68,7 @@ linux-rel-nogate:
   - ./mach clean-nightlies --keep 3 --force
   - env CC=gcc-5 CXX=g++-5 ./mach build --release
   - python ./etc/ci/chaos_monkey_test.py
+  - bash ./etc/ci/mutation_test.sh
 
 mac-rel-intermittent:
   - ./mach clean-nightlies --keep 3 --force

etc/ci/mutation_test.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+PS1="" source python/_virtualenv/bin/activate
+# `PS1` must be defined before activating virtualenv
+python python/servo/mutation/init.py components/script/dom