[PATCH] replace sha1sum with sum in test suite for portability

Replace sha1sum with sum in t1002 for portability.

This patch replaces sha1sum because some systems do not have GNU coreutils "out of the
box" and not having sha1sum will make the test fail.  I chose to replace sha1sum with sum
(after considering several different checksumming methods) because by default GNU sum
runs in a "BSD compatibility mode" so it should Just Work across OSes as long as its
installed.

Signed-off-by: Mark Allen <redacted>