[meson] make 'tests' option enabled by default

Not sure what 'auto' can mean here but it makes my local test to not run properly
diff --git a/meson_options.txt b/meson_options.txt
index fa72e9d..6fe06df 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -21,7 +21,7 @@
   description: 'Enable CoreText shaper backend on macOS')
 
 # Common feature options
-option('tests', type : 'feature', value : 'auto', yield : true,
+option('tests', type : 'feature', value : 'enabled', yield : true,
   description: 'Enable or disable unit tests')
 option('introspection', type : 'feature', value : 'disabled', yield : true,
   description : 'Generate gobject-introspection bindings (.gir/.typelib files)')