runtime: fix preemption deadlocks in TestDebugCall*

TestDebugCall* uses atomic spin loops and hence can deadlock if the
garbage collector is enabled (because of #10958; ironically,
implementing debugger call injection is closely related to fixing this
exact issue, but we're not there yet).

Fix this by disabling the garbage collector during these tests.

Updates #25519 (might fix it, though I suspect not)

Change-Id: If1e454b9cdea8e4b1cd82509b762c75b6acd8476
Reviewed-on: https://go-review.googlesource.com/114086


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>