tests: make 'test_oid' print trailing newline

Unlike other test helper functions, 'test_oid' doesn't terminate its
output with a LF, but, alas, the reason for this, if any, is not
mentioned in 2c02b110da (t: add test functions to translate
hash-related values, 2018-09-13)).

Now, in the vast majority of cases 'test_oid' is invoked in a command
substitution that is part of a heredoc or supplies an argument to a
command or the value to a variable, and the command substitution would
chop off any trailing LFs, so in these cases the lack or presence of a
trailing LF in its output doesn't matter.  However:

  - There appear to be only three cases where 'test_oid' is not
    invoked in a command substitution:

      $ git grep '\stest_oid ' -- ':/t/*.sh'
      t0000-basic.sh:  test_oid zero >actual &&
      t0000-basic.sh:  test_oid zero >actual &&
      t0000-basic.sh:  test_oid zero >actual &&

    These are all in test cases checking that 'test_oid' actually
    works, and that the size of its output matches the size of the
    corresponding hash function with conditions like

      test $(wc -c <actual) -eq 40

    In these cases the lack of trailing LF does actually matter,
    though they could be trivially updated to account for the presence
    of a trailing LF.

  - There are also a few cases where the lack of trailing LF in
    'test_oid's output actually hurts, because tests need to compare
    its output with LF terminated file contents, forcing developers to
    invoke it as 'echo $(test_oid ...)' to append the missing LF:

      $ git grep 'echo "\?$(test_oid ' -- ':/t/*.sh'
      t1302-repo-version.sh:  echo $(test_oid version) >expect &&
      t1500-rev-parse.sh:     echo "$(test_oid algo)" >expect &&
      t4044-diff-index-unique-abbrev.sh:      echo "$(test_oid val1)" > foo &&
      t4044-diff-index-unique-abbrev.sh:      echo "$(test_oid val2)" > foo &&
      t5313-pack-bounds-checks.sh:    echo $(test_oid oidfff) >file &&

    And there is yet another similar case in an in-flight topic at:

      https://public-inbox.org/git/813e81a058227bd373cec802e443fcd677042fb4.1670862677.git.gitgitgadget@gmail.com/

Arguably we would be better off if 'test_oid' terminated its output
with a LF.  So let's update 'test_oid' accordingly, update its tests
in t0000 to account for the extra character in those size tests, and
remove the now unnecessary 'echo $(...)' command substitutions around
'test_oid' invocations as well.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4044-diff-index-unique-abbrev.sh

@@ -34,12 +34,12 @@ test_expect_success 'setup' '
 	100644 blob $(test_oid hash2)	foo
 	EOF
 
-	echo "$(test_oid val1)" > foo &&
+	test_oid val1 > foo &&
 	git add foo &&
 	git commit -m "initial" &&
 	git cat-file -p HEAD: > actual &&
 	test_cmp expect_initial actual &&
-	echo "$(test_oid val2)" > foo &&
+	test_oid val2 > foo &&
 	git commit -a -m "update" &&
 	git cat-file -p HEAD: > actual &&
 	test_cmp expect_update actual