build: test: Fix missing variable

This was incorrectly copied from another repo.

Change-Id: I6ca56fef6205733f86f053110e06481b53636677
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/test/meson.build b/test/meson.build
index 552fb01..5ef050b 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