meson: improve test handling

Tests will now be built at `meson test`-time, so there's no cost if the
user isn't going to run them.

This is better than guarding on build type, as distros may want to test production
builds before shipping.

Signed-off-by: Sam James <sam@gentoo.org>

meson.build

@@ -317,7 +317,4 @@ if get_option('build_apps') and host_machine.system() != 'windows'
   subdir('apps')
 endif
 
-# Optional tests
-if get_option('buildtype') == 'debug'
-  subdir('tests')
-endif 
+subdir('tests')