meson: Enable tests by default

The test subdirectory should no longer be compiled when `tests`
disabled, So tests should be explicitly enabled by default.
Also `tests` has been disabled in meta-phosphor[1].

[1] https://gerrit.openbmc.org/c/openbmc/openbmc/+/43812

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I80352925719d3be137f99457ff38a948413f9c17
diff --git a/meson_options.txt b/meson_options.txt
index b7869f4..dda6d81 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -1,3 +1,5 @@
 option(
-    'tests', type: 'feature', description: 'Build tests.',
+    'tests', type: 'feature',
+    value: 'enabled',
+    description: 'Build tests.'
 )