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: I3cc28241cd9ef0d907aa73409a63a4470a628e70
diff --git a/meson_options.txt b/meson_options.txt
index f862dfd..d626a88 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -1,5 +1,6 @@
 option(
     'tests', type : 'feature',
+    value: 'enabled',
     description : 'Build tests'
 )