meson.build: disable tests when cross-compiling

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
4 jobs for cross-compile-support in 8 minutes and 23 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Build
passed #266691
fedora-x86_64-meson

00:01:48

passed #266692
fedora-x86_64-python3.5

00:01:42

passed #266693
win32
msys2-mingw32-meson

00:03:27

passed #266694
win32
vs2017-x64-meson

00:05:00