Spawn a code-server instance for each test suite

This uses the current dev build by default but can be overidden with
CODE_SERVER_TEST_ENTRY (for example to test a release or some other
version).

Each instance has a separate state directory. This should make
parallelization work.

This also means you are no longer required to specify the password and
address yourself (or the extension directory once we add a test
extension). `yarn test:e2e` should just work as-is.

Lastly, it means the tests are no longer subject to yarn watch randomly
restarting.

test/playwright.config.ts

@@ -6,8 +6,7 @@ import path from "path"
 const config: PlaywrightTestConfig = {
   testDir: path.join(__dirname, "e2e"), // Search for tests in this directory.
   timeout: 60000, // Each test is given 60 seconds.
-  retries: process.env.CI ? 2 : 1, // Retry twice in CI due to flakiness.
-  workers: 1,
+  retries: process.env.CI ? 2 : 1, // Retry in CI due to flakiness.
   globalSetup: require.resolve("./utils/globalSetup.ts"),
   reporter: "list",
   // Put any shared options on the top level.