Use the serial automake test harness

Once upon a time, automake had one way to run tests. Apparently this is
(nowadays?) called the serial test harness. Then, in some release (I do
not remember which one), the parallel test harness became the default.

The parallel harness runs all tests in parallel, but does not (really)
show the test output, but instead has output redirected to files. Sort
of. I did not find the result of my printf() anywhere.

The automake docs strongly discourage using the serial test harness [0],
but do not really say why. For cairo, I do not see problems:

We have some quick, small checks (e.g. is cairo_public used where needed
in the public headers). And then there is the big, heavy-weight test
suite (cairo-test-suite). Thus, running these things in parallel has
basically no benefits.

Additionally, cairo-test-suite takes really, really long. Not seeing any
output while it is running is annoying. Failing to find the output even
in files is bad.

Thus, since I do not see any benefits and only downsides to the parallel
test harness, this commit switches to the serial one.

[0]: https://www.gnu.org/software/automake/manual/html_node/Serial-Test-Harness.html

Signed-off-by: Uli Schlachter <psychon@znc.in>
7 jobs for serial-test-harness in 24 minutes and 35 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prep
passed #7739602
fedora image

00:00:15

 
  Test
passed #7739603
fedora autotools build

00:24:19

passed #7739604
fedora meson build

00:00:28

passed #7739607
meson android arm64 fedora

00:02:17

passed #7739608
gst-macos-11.1
meson macOS

00:03:17

passed #7739605
docker windows 1809
meson vs2017 amd64

00:03:02

passed #7739606
docker windows 1809
meson vs2017 x86

00:02:56