build: use allowed over enabled

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`.

Change-Id: I882edd53856df38c8292bc3313b9342b2b1ceddd
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/tests/meson.build b/tests/meson.build
index 28915d6..3582c07 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -13,7 +13,7 @@
         gmock_dep = gtest_proj.dependency('gmock')
     else
         assert(
-            not get_option('tests').enabled(),
+            not get_option('tests').allowed(),
             'Googletest is required if tests are enabled'
         )
     endif