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.

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