meson: allow disabling tests

The new meson build system support diverges from former build system
where the tests were not built until required.

This has caused an issue in NixOS[1] due to broken build of tests in
Darwin platform, so this patch allows the control if tests should be
build or not.

1. https://github.com/NixOS/nixpkgs/pull/235210

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

meson.build

@@ -134,8 +134,8 @@ if not meson.is_cross_build()
   if py.found() and swig.found()
     subdir('pylibfdt')
   endif
+endif
 
-  if get_option('tools')
+if get_option('tests')
   subdir('tests')
 endif
-endif

meson_options.txt

@@ -10,3 +10,5 @@ option('python', type: 'feature', value: 'auto',
        description: 'Build pylibfdt Python library')
 option('static-build', type: 'boolean', value: false,
        description: 'Build static binaries')
+option('tests', type: 'boolean', value: true,
+       description: 'Build tests')