Tests: Update infrastructure & add travis config (#100)

* Tests: Move tests infrastructure, set up travis

Consolidates the composer files so we can run install once in travis.

* Return empty array if no values are $_POST'ed to the form

Fixes an issue with tests + null values in logs

* Clean up some naming & create helper functions for tests

* Debug travis

* Remove debug

* Fix notices on form

* Turn on multisite flag

.travis.yml

@@ -0,0 +1,16 @@
+sudo: required
+dist: trusty
+language: php
+php: 7.2
+env: WP_VERSION=latest WP_MULTISITE=1 WP_CORE_DIR=/tmp/wordpress
+
+install:
+  - bash bin/install-wp-tests.sh wporg_5ftf_test root '' localhost $WP_VERSION
+  - composer install
+
+script:
+  - composer run-script test -- --version
+  - composer run-script test -- -c phpunit.xml.dist
+  - touch $TRAVIS_BUILD_DIR/tmp.php
+  - export CHANGED_FILES=$(git diff --name-only --diff-filter=AM $TRAVIS_BRANCH...HEAD | tr '\n' ' ')
+  - composer run-script phpcs $TRAVIS_BUILD_DIR/tmp.php $(echo $CHANGED_FILES) -- -n