mailmap tests: modernize syntax & test idioms

mailmap tests: modernize syntax & test idioms

Refactor the mailmap tests to:

 * Setup "actual" test files in the body of "test_expect_success"

 * Don't have X of "test_expect_success X Y" be an unquoted string.

 * Not to carry over test config between tests, and instead use
   "test_config".

 * Replace various "echo" a line-at-a-time patterns with here-docs.

 * Change a case of "log.mailmap=False" to use the lower-case
   "false". Both work, but this ends up in git-config's boolean
   parsing and these atypical values are tested for elsewhere. Let's
   use the lower-case to not draw the reader's attention to this
   abnormality.

 * Remove commentary asserting that things work a given way in favor
   of simply testing for it, i.e. in the case of a .mailmap file
   outside of the repository.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>