fix(tests): Prevent router_test.exs from running async

[joshlarson]

Most tests rely on the HOST environment variable to be set to nil, so that it defaults to localhost. If HOST is set to www.mbta.com, that causes a redirect. router_test.exs has a test that validates that redirect, by setting HOST to www.mbta.com, validating the redirect, and then setting it back to nil.

This is what that normally looks like in context:

sequenceDiagram
  RouterTest->>Env: set_env("HOST", "www.mbta.com")
  RouterTest->>Env: get_env("HOST")
  Env-->>RouterTest: "www.mbta.com"
  RouterTest->>Env: set_env("HOST", nil)
  OtherTest->>Env: get_env("HOST")
  Env-->>OtherTest: nil

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(Note the existence of OtherTest, which also retrieves HOST, and relies on HOST being nil.)

BUT, when router_test.exs's execution is interleaved with other tests, then sometimes another test will try to retrieve HOST (well, another test will invoke a conn, which then invokes HOST through the CanonicalHostname plug) before router_test.exs has set HOST back to nil.

sequenceDiagram
  RouterTest->>Env: set_env("HOST", "www.mbta.com")
  RouterTest->>Env: get_env("HOST")
  Env-->>RouterTest: "www.mbta.com"
  OtherTest->>Env: get_env("HOST")
  Env-->>OtherTest: "www.mbta.com"
  RouterTest->>Env: set_env("HOST", nil)

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The conn invoked by OtherTest gets the www.mbta.com HOST and does a redirect, which the other test wasn't expecting, and fails.

This creates a very challenging debugging environment, because, in this context, OtherTest could refer to literally any other test that makes an HTTP request or does anything with a conn. This bug was causing very rare non-deterministic failures in every other HTTP-related test.

The solution here is to remove the async: true flag from router_test.exs, preventing it from interleaving itself with other tests.

Hat-tip to @thecristen for helping me figure out that this CanonicalHostname plug was the culprit.