Adjusted the test types/range/bradc/overflowInComputeBlock.chpl.
Merge pull request #780 from vasslitvinov/update-overflowInComputeBlock.chpl

Adjusted the test types/range/bradc/overflowInComputeBlock.chpl

The test was created to test the fix in r18647 aka SHA1 2287215.
Back then the default integers were 32-bit, and I wasn't sure
that today this test is useful.

I changed the test to use 16-bit integers explicitly,
and verified that it fails if I revert that fix.
So, in this sense, this test IS useful today.

Also I am removing the .skipif that was added recently
to avoid valgrind timeout for this test - it should not
be an issue given this smaller problem size.