meson: Default tests feature to 'auto'

With that, reduce the strength of the get_option('tests') conditions to
allowed().

Change-Id: I2ae42606a64d39c2bc5667bf009c95558c0bc4d6
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/meson.build b/meson.build
index 86b2250..529884b 100644
--- a/meson.build
+++ b/meson.build
@@ -48,7 +48,7 @@
 subdir('include')
 subdir('src')
 
-if get_option('tests').enabled()
+if get_option('tests').allowed()
   subdir('tests')
 
   c_suite = meson.get_compiler('c').get_id()