test: make issue54343.go robust against the tiny allocator

I structured the test for issue54343.go after issue46725.go, where I
was careful to use `[4]int`, which is a type large enough to avoid the
tiny object allocator (which interferes with finalizer semantics). But
in that test, I didn't note the importance of that type, so I
mistakenly used just `int` in issue54343.go.

This CL switches issue54343.go to use `[4]int` too, and then adds
comments to both pointing out the significance of this type.

Updates #54343.

Change-Id: I699b3e64b844ff6d8438bbcb4d1935615a6d8cc4
Reviewed-on: https://go-review.googlesource.com/c/go/+/423115
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>

test/fixedbugs/issue46725.go

@@ -8,7 +8,7 @@ package main
 
 import "runtime"
 
-type T [4]int
+type T [4]int // N.B., [4]int avoids runtime's tiny object allocator
 
 //go:noinline
 func g(x []*T) ([]*T, []*T) { return x, x }

test/fixedbugs/issue54343.go

@@ -26,7 +26,7 @@ func New[X any]() *T[X] {
 	return p
 }
 
-type T[X any] int
+type T[X any] [4]int // N.B., [4]int avoids runtime's tiny object allocator
 
 func (*T[X]) M() {}