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: Id589cab6f0a0b49365bc4f6b8a151243fde02dba
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/bmc/meson.build b/bmc/meson.build
index 0ff1c17..62fbeec 100644
--- a/bmc/meson.build
+++ b/bmc/meson.build
@@ -260,7 +260,7 @@
 
 # If test coverage of source files within the root directory are wanted,
 # need to define and build the tests from here
-if not build_tests.disabled()
+if build_tests.allowed()
     gtest = dependency('gtest', main: true, disabler: true, required: build_tests)
     include_srcs = declare_dependency(sources: [
         'utils.cpp',