build: Don't build tests by default in submodules

Change-Id: I072c2dae0dc94d66c29296ef2cf8e6943451abcc
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/meson.build b/meson.build
index 3a121be..c7758e9 100644
--- a/meson.build
+++ b/meson.build
@@ -5,6 +5,7 @@
       'cpp_std=c++20',
       'warning_level=3',
       'werror=true',
+      'tests=' + (meson.is_subproject() ? 'disabled' : 'auto'),
     ],
     version: '1.0.0',
 )