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: I37fcb391dcb215a86a60c26f7512f6ccb8312dc4
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/phosphor-regulators/meson.build b/phosphor-regulators/meson.build
index cd2c39c..a9a3dd5 100644
--- a/phosphor-regulators/meson.build
+++ b/phosphor-regulators/meson.build
@@ -4,6 +4,6 @@
 
 subdir('src')
 
-if get_option('tests').enabled()
+if get_option('tests').allowed()
     subdir('test')
 endif