This fixes a few different issues that led to hangs and general
flakiness in the TestDebugCall* tests.

1. This fixes missing wake-ups in two error paths of the SIGTRAP
   signal handler. If the goroutine was in an unknown state, or if
   there was an unknown debug call status, we currently don't wake the
   injection coordinator. These are terminal states, so this resulted
   in a hang.

2. This adds a retry if the target goroutine is in a transient state
   that prevents us from injecting a call. The most common failure
   mode here is that the target goroutine is in _Grunnable, but this
   was previously masked because it deadlocked the test.

3. Related to 2, this switches the "ready" signal from the target
   goroutine from a blocking channel send to a non-blocking channel
   send. This makes it much less likely that we'll catch this
   goroutine while it's in the runtime performing that send.

4. This increases GOMAXPROCS from 2 to 8 during these tests. With the
   current setting of 2, we can have at most the non-preemptible
   goroutine we're injecting a call in to and the goroutine that's
   trying to make it exit. If anything else comes along, it can
   deadlock. One particular case I observed was in TestDebugCallGC,
   where runtime.GC() returns before the forEachP that prepares
   sweeping on all goroutines has finished. When this happens, the
   forEachP blocks on the non-preemptible loop, which means we now
   have at least three goroutines that need to run.

Fixes #25519.

Updates #29124.

Change-Id: I7bc41dc0b865b7d0bb379cb654f9a1218bc37428
Reviewed-on: https://go-review.googlesource.com/c/154112
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>