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`.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: Ie729807e5e08b904f8c677aead222d5965e13a04
diff --git a/meson.build b/meson.build
index 0fe0a6a..327d2d3 100644
--- a/meson.build
+++ b/meson.build
@@ -35,7 +35,7 @@
 
 subdir('src/rde')
 subdir('src')
-if not get_option('tests').disabled()
+if get_option('tests').allowed()
   subdir('test')
 endif