Convert integration tests to be self-hosted

* Reconfigure xo and eslint to purposely ignore bad configurations
* Rewrite expectations to use snapshots
* Fix trailing symbols strip functionality for output cleanup
* Use posix path relative resolution on tests filename snapshots
* Normalize posix paths on test snapshots

Author: jonathansamines

test-tap/fixture/environment-variables/ava.config.js

@@ -1,4 +1,4 @@
-const test = require('../../..');
+const test = require('ava');
 
 test('works', t => {
 	t.pass();

test-tap/fixture/pkg-conf/resolve-dir/dir-a-wrapper/dir-a/dir-a-wrapper-3.js

@@ -0,0 +1,7 @@
+{
+	"ava": {
+		"files": [
+			"*.js"
+		]
+	}
+}