test/recover4.go: use mprotect to create a hole instead of munmap

Currently the recover4 test, which recovers from a panic created from a
fault, generates a fault by creating a hole in a mapping. It does this
via munmap. However, it's possible the runtime can create a new mapping
that ends up in that hole, for example if the GC executes, causing the
test to fail.

In fact, this is the case now with a smaller minimum heap size.

Modify the test to use mprotect, and clean up the code a little while
we're here: define everything in terms of the length of original
mapping, deduplicate some constants and expressions, and have the test
recover properly even if recover() returns nil (right now it panics
because it fails to type assert nil as error).

Fixes #49381.

Change-Id: If399eca564466e5e8aeb2dc6f86a246d0fce7b5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/363534


Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>