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: I18a2a8f7607bda00fc7ebc8855884fe5f55d9e7d
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/tests/meson.build b/tests/meson.build
index 2cadb5c..1fce748 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -21,7 +21,7 @@
 
 unit_tests = []
 
-if get_option('tests').enabled()
+if get_option('tests').allowed()
     # generate the test executable
     foreach unit_test : unit_tests
         test(