[fuzzer][test] Avoid big-file-copy.test on memory constrained devices

The test "big-file-copy.test" introduced in D146189 and constrained to darwin by
D147094, is by this differential further constrained to only those devices with
sufficient resources. Also correct the test to read the environment variable
"result" from the same shell in which it was stored (which may differ on devices).

Reviewed By: thetruestblue

Differential Revision: https://reviews.llvm.org/D147502