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: I96a9fac0a19ea6d82a895a49200e81874879d2e4
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 527be0c..061c7e1 100644
--- a/meson.build
+++ b/meson.build
@@ -83,7 +83,7 @@
 
 build_tests = get_option('tests')
 
-if not build_tests.disabled()
+if build_tests.allowed()
     subdir('test')
 endif