build-tests: The allowed method is used by default

The allowed method returns true when the feature is set to `enabled`
or `auto`.
The allowed method returns false when the feature is set to
`disabled`.
So we prefer to use the `allowed` method instead of `enabled`.

Change-Id: I1f1bb8bf40c1312d6943e67a81abc58dabfd94cd
Signed-off-by: George Liu <liuxiwei@inspur.com>
diff --git a/src/meson.build b/src/meson.build
index e34838c..4383d72 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -56,7 +56,7 @@
 )
 
 build_tests = get_option('tests')
-if not build_tests.disabled()
+if build_tests.allowed()
   subdir('test')
 endif