Specify a concrete stack size in channel tests

The channel-stack-overflow-issue-102246 regression test fails on
platforms with a small default stack size (e.g. Fuchsia, with a default
of 256KiB). Update the test to specify an exact stack size for both the
sender and receiver operations, to ensure it is platform agnostic.

Set the stack size to less than the total allocation size of the mpsc
channel, to continue to prove that the allocation is on the heap.

Author: Jeff-A-Martin

tests/ui/std/channel-stack-overflow-issue-102246.rs

@@ -10,20 +10,30 @@
 // Ref: https://github.com/rust-lang/rust/issues/102246
 
 use std::sync::mpsc::channel;
-use std::thread;
+use std::thread::Builder;
 
 const N: usize = 32_768;
+const SLOTS: usize = 32;
+// Use a stack size that's smaller than N * SLOTS, proving the allocation is on the heap.
+//
+// The test explicitly specifies the stack size, because not all platforms have the same default
+// size.
+const STACK_SIZE: usize = (N*SLOTS) - 1;
+
 struct BigStruct {
     _data: [u8; N],
 }
 
 fn main() {
     let (sender, receiver) = channel::<BigStruct>();
 
-    let thread1 = thread::spawn(move || {
+    let thread1 = Builder::new().stack_size(STACK_SIZE).spawn(move || {
         sender.send(BigStruct { _data: [0u8; N] }).unwrap();
-    });
-
+    }).expect("thread1 should spawn successfully");
     thread1.join().unwrap();
-    for _data in receiver.try_iter() {}
+
+    let thread2 = Builder::new().stack_size(STACK_SIZE).spawn(move || {
+        for _data in receiver.try_iter() {}
+    }).expect("thread2 should spawn successfully");
+    thread2.join().unwrap();
 }