Fix run-test.sh to work with BSD mktemp

Avoid the GNU-specific --tmpdir flag to mkdir and always prefer to use
the directory specified in TMPDIR rather than /tmp.

See fontconfig/fontconfig#288
11 jobs for ryandesign-run-test-bsd in 8 minutes and 8 seconds (queued for 9 seconds)
Status Name Job ID Coverage
  Build
passed meson android arm64 fedora #11800131

00:01:17

passed meson macos #11800129
gst-macos-10.15

00:03:06

passed meson-mingw-w64-build #11800126

00:06:28

manual meson msys2 #11800130
docker windows 1809 allowed to fail manual
passed meson-shared-build #11800124

00:05:57

passed meson-static-build #11800125

00:05:56

passed meson vs2017 amd64 #11800127
docker windows 1809

00:01:17

passed meson vs2017 x86 #11800128
docker windows 1809

00:01:14

passed mingw-build #11800123

00:06:51

passed shared-build #11800121

00:08:07

passed static-build #11800122

00:06:25