Makefile: use mkdir_p_parent_template for UNIT_TEST_BIN

Makefile: use mkdir_p_parent_template for UNIT_TEST_BIN

We build the UNIT_TEST_BIN directory (t/unit-tests/bin) on the fly with
"mkdir -p". And so the recipe for UNIT_TEST_PROGS, which put their
output in that directory, depend on UNIT_TEST_BIN to make sure it's
there.

But using a normal dependency leads to weird outcomes, because the
timestamp of the directory is important. For example, try this:

  $ make
  [...builds everything...]

  [now re-build one unit test]
  $ touch t/unit-tests/t-ctype.c
  $ make
      SUBDIR templates
      CC t/unit-tests/t-ctype.o
      LINK t/unit-tests/bin/t-ctype

So far so good. Now running make again should build nothing. But it
doesn't!

  $ make
      SUBDIR templates
      LINK t/unit-tests/bin/t-basic
      LINK t/unit-tests/bin/t-mem-pool
      LINK t/unit-tests/bin/t-strbuf

Er, what? Let's rebuild again:

  $ make
      SUBDIR templates
      LINK t/unit-tests/bin/t-ctype

Weird. And now we ping-pong back and forth forever:

  $ make
      SUBDIR templates
      LINK t/unit-tests/bin/t-basic
      LINK t/unit-tests/bin/t-mem-pool
      LINK t/unit-tests/bin/t-strbuf
  $ make
      SUBDIR templates
      LINK t/unit-tests/bin/t-ctype

What happens is that writing t/unit-tests/bin/t-ctype updates the mtime
of the directory t/unit-tests/bin. And then on the next invocation of
make, all of those other tests are now older and so get rebuilt. And
back and forth forever.

We can fix this by making the directory as part of the build recipe for
the programs, using the template from 0b6d0bc924 (Makefiles: add and use
wildcard "mkdir -p" template, 2022-03-03).

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