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: I0569970ebf98929b443aeff84a60e66c046367b8
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 3ac632d..0ac1b3b 100644
--- a/meson.build
+++ b/meson.build
@@ -75,6 +75,6 @@
     subdir('vendor-example')
 endif
 
-if not build_tests.disabled()
+if build_tests.allowed()
     subdir('test')
 endif