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: I079b8c0c94e9e22b37f84344cd8e6638e47aa0bc
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index bd19af9..11bad07 100644
--- a/meson.build
+++ b/meson.build
@@ -15,7 +15,7 @@
 libbej_incs = include_directories('include', 'include/libbej')
 subdir('src')
 subdir('include/libbej')
-if not get_option('tests').disabled()
+if get_option('tests').allowed()
   subdir('test')
 endif