t6006, t6012: adjust tests to use 'setup' instead of synonyms

With the new ability to pass --run=setup to select which tests to run,
it is more convenient if tests use the term "setup" instead of synonyms
like 'prepare' or 'rebuild'.  There are undoubtedly many other tests in
our testsuite that could be changed over too, these are just a couple
that I ran into.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6006-rev-list-format.sh

@@ -339,7 +339,7 @@ commit $head1
 .. (hinzugef${added_utf8_part_iso88591}gt) foo
 EOF
 
-test_expect_success 'prepare expected messages (for test %b)' '
+test_expect_success 'setup expected messages (for test %b)' '
 	cat <<-EOF >expected.utf-8 &&
 	commit $head3
 	This commit message is much longer than the others,

t/t6012-rev-list-simplify.sh

@@ -168,7 +168,7 @@ test_expect_success '--full-diff is not affected by --parents' '
 #
 # This example is explained in Documentation/rev-list-options.txt
 
-test_expect_success 'rebuild repo' '
+test_expect_success 'setup rebuild repo' '
 	rm -rf .git * &&
 	git init &&
 	git switch -c topic &&