build: use allowed over enabled or not-disabled

Meson feature options are typically in a tri-state of enabled, disabled,
or auto.  The enabled and disabled functions on an option (from
`get_option`) no longer return true for auto features.  Instead, the
expectation is to use `allowed()` which is true for both enabled and
auto.

Switch all uses of `enabled` to `allowed`.
Switch all uses of `not disabled` to `allowed`.

Change-Id: Iee6d148c2c6717c8e925dfee65f557c55b4c2b67
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index a660425..4771723 100644
--- a/meson.build
+++ b/meson.build
@@ -11,6 +11,6 @@
   ])
 
 subdir('src')
-if not get_option('tests').disabled()
+if get_option('tests').allowed()
   subdir('test')
 endif