Always run-test-conf, but skip if not built.

The test-conf test requires libjson-c to be available in order to be
built. However, there has been no user indication that additional tests
could be built if the json-c development files were available.

Continue to not build test-conf if json-c is not available, but do run
the test harness. The test harness is updated to SKIP the test if the
test-conf binary is unavailable.
10 jobs for always_try_running_test_conf in 7 minutes and 12 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #6195962
gst-macos-10.15
meson macos

00:03:20

passed #6195959
meson-mingw-w64-build

00:03:17

manual #6195963
docker windows 1809 allowed to fail manual
meson msys2
passed #6195957
meson-shared-build

00:05:49

passed #6195958
meson-static-build

00:05:02

passed #6195960
docker windows 1809
meson vs2017 amd64

00:01:26

passed #6195961
docker windows 1809
meson vs2017 x86

00:01:25

passed #6195956
mingw-build

00:05:21

passed #6195954
shared-build

00:07:12

passed #6195955
static-build

00:03:46