test: refactor integration test for LSP

It is not always straight-forward to write unit tests for the code in
this repo because of the nature of the language server.

However, since the language server uses JSON-RPC, it makes sense to test
the server along this boundary through stateful integration test.

This PR makes writing integration tests a lot easier by refactoring the
test client. As a result, we can now

1. Use strongly typed requests and responses.
2. Have much better abstraction of the test client.
3. Easily write more tests!

This refactoring is done in preparation for the upcoming ngcc feature.

Author: kyliau

integration/lsp/jasmine.json

@@ -0,0 +1,6 @@
+{
+  "spec_dir": "integration/out/lsp",
+  "spec_files": [
+    "*_spec.js"
+  ]
+}