This fixes #1015.

When Start() is called, it passes StartTimeout to the relayer goroutine,
which is monitoring the Stop signal, as well as the OS signal handler.

It then exits the goroutine if the start context expires, which would
naturally happen if the user uses the default setting (15s timeout) or
any finite amount of timeout value.

This causes the app to then be not responsive to any OS signals such as
SIGTERM or SIGINT.

This was a regression introduced in 1.19 release via #989 .

To fix this, this commit simply removes the relayer goroutine from
selecting on the start context being completed.

Verified that all tests are still passing without any goroutine leak,
except one test that was triggering a panic to test the panic handler.

To prevent that one test from opting out every single test into
goleak.VerifyNone(t) in every sub test, I pulled out that panic test
into a separate test package so that we can continue to use
goleak.VerifyNone() method in app_test.

Starting from 1.19, calling Stop() when an app has been stopped returns
an error.

This actually is an unexpected change for many users who have been using
fxtest for testing their apps. Specifically, if they have a server that
errors out when tests are written, these will often trigger Stop() to be
called many times..