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: I53f6f684a474e85379503819a2246b888b6a85f8
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 1be93b4..a22ec18 100644
--- a/meson.build
+++ b/meson.build
@@ -176,6 +176,6 @@
         install_dir: get_option('bindir')
     )
 
-if not get_option('tests').disabled()
+if get_option('tests').allowed()
   subdir('test')
 endif