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: Ia2c951e0b40131650ee8fd1d7f76a322686a3898
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index a5320f7..77e31df 100644
--- a/meson.build
+++ b/meson.build
@@ -60,6 +60,6 @@
   install_dir: systemd.get_variable('systemdsystemunitdir'))
 
 build_tests = get_option('tests')
-if not build_tests.disabled()
+if build_tests.allowed()
   subdir('test')
 endif