buf tests: allocate a smaller size for the oom

On Linux (where we run valgrind) allocate a smaller buffer, but still an
insanely large size.  This will cause malloc to fail but will not cause
valgrind to report a likely error with a negative-sized malloc.

Keep the original buffer size on non-Linux platforms: this is
well-tested on them and changing it may be problematic.  On macOS, for
example, using the new size causes `malloc` to print a warning to
stderr.

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