build: Use a boolean default for a boolean option, rather than a string

Meson 1.1.0 officially deprecates string defaults for boolean options,
but boolean defaults worked in many older Meson versions, going back to
at least 0.49.x.

Signed-off-by: Simon McVittie <smcv@collabora.com>

meson_options.txt

@@ -5,5 +5,5 @@ option(
   'tests',
   type : 'boolean',
   description : 'build and run unit tests',
-  value : 'true',
+  value : true,
 )