Infrastructure: Run regression tests in parallel in TravisCI and improve test selection logic (pull #1465)

Fixes issues #1449 and 1450 by running regression tests in parallel and improving logic that determines which tests to run.

To run in parallel:
1. Updates `.travis.yml` to use the [Ava feature](https://github.com/avajs/ava#parallel-runs-in-ci) that allows tests in parallel across three VMs in the CI
2. Updates Ava version to include [this bug fix](avajs/ava#2461) -- the fix prevents a false error when Ava tries to parallelize less than three test files (it parallelizes by files, so one test file is not parallelized).
3. Updates the script that runs the regression tests in the CI to only the test files for the example or test file you are editing to take advantage of the parallelization option. Before, it filtered tests by using a regex on the name of the test. Now, it explicitly lists all the test FILES that should be run.

When running `bash regression-tests.sh` locally from a branch that is not master, the script will now run as if the current branch is a pull request against master.
You can now test locally that file changes in your branch will trigger the correct tests.
The logic is:
1. If `package.json` has been edited, or if test utility files have been edited, or if example utility files have been edited, then all regression tests will be run in the CI.
2. Otherwise, if an example is edited, then the regression tests for all of the examples contained in the same example directory are run. For example: if you edit one of the two checkbox examples, then both the checkbox-1 and checkbox-2 example regression tests will be run in the CI.
3. All test files that have been edited will be run in the CI.

Author: spectranaut

.travis.yml

@@ -7,6 +7,7 @@ git:
   depth: 3
 
 stages:
+- Lint
 - Test
 - Deploy
 
@@ -16,30 +17,52 @@ jobs:
     - env: ALLOW_FAILURE=true
 
   include:
-    - stage: Test
+    - stage: Lint
       name: CSS Linting
       script: npm run lint:css
-    - stage: Test
+    - stage: Lint
       name: JS Linting
       script: npm run lint:es
-    - stage: Test
+    - stage: Lint
       name: HTML Linting
       script: npm run vnu-jar
-    - stage: Test
+    - stage: Lint
       name: Spellcheck
       script: npm run lint:spelling
       env: ALLOW_FAILURE=true
+    - stage: Lint
+      name: Regression Tests Coverage Report
+      script: node test/util/report.js
+      env: ALLOW_FAILURE=true
 
     - stage: Test
       name: AVA Regression Tests
       addons:
         firefox: latest
       script: scripts/regression-tests.sh
-      env: TEST_WAIT_TIME=1000
+      env:
+        - TEST_WAIT_TIME=1000
+        - CI_NODE_TOTAL=3
+        - CI_NODE_INDEX=0
     - stage: Test
-      name: Regression Tests Coverage Report
-      script: node test/util/report.js
-      env: ALLOW_FAILURE=true
+      name: AVA Regression Tests
+      addons:
+        firefox: latest
+      script: scripts/regression-tests.sh
+      env:
+        - TEST_WAIT_TIME=1000
+        - CI_NODE_TOTAL=3
+        - CI_NODE_INDEX=1
+    - stage: Test
+      name: AVA Regression Tests
+      addons:
+        firefox: latest
+      script: scripts/regression-tests.sh
+      env:
+        - TEST_WAIT_TIME=1000
+        - CI_NODE_TOTAL=3
+        - CI_NODE_INDEX=2
+
     - stage: Deploy
       if: branch = master AND type != pull_request
       script: skip