t1006: update 'run_tests' to test generic object specifiers

t1006: update 'run_tests' to test generic object specifiers

Update the 'run_tests' test wrapper so that the first argument may refer to
any specifier that uniquely identifies an object (e.g. a ref name,
'<OID>:<path>', '<OID>^{<type>}', etc.), rather than only a full object ID.

Also add tests that use non-OID identifiers, ensuring appropriate parsing in
'cat-file'. The identifiers used in some of the added tests include a space,
which is incompatible with the '%(rest)' atom. To accommodate that without
removing the test case, use 'test_expect_failure' when 'object_name'
includes a space.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>