t1006: modernize test style to use `test_cmp`

t1006: modernize test style to use `test_cmp`

The tests for git-cat-file(1) are quite old and haven't ever been
updated since they were introduced. They thus tend to use old idioms
that have since grown outdated. Most importantly, many of the tests use
`test $A = $B` to compare expected and actual output. This has the
downside that it is impossible to tell what exactly is different between
both versions in case the test fails.

Refactor the tests to instead use `test_cmp`. While more verbose, it
both tends to be more readable and will result in a nice diff in case
states don't match.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>