build: test: Fix invalid variable

We don't define the build_tests variable, reference the option directly
instead.

Change-Id: Ic190c083c2c4ebd4c1b3715cea39b563e0294b6d
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/test/meson.build b/test/meson.build
index cb9d8cc..0faf701 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -16,7 +16,7 @@
       ])
     gmock = gtest_proj.dependency('gmock')
   else
-    assert(not build_tests.enabled(), 'Googletest is required')
+    assert(not get_option('tests').enabled(), 'Googletest is required')
   endif
 endif