Do not build tests in release mode

Assertions would not be executed.

And as a side effect, it causes "unused variable" warnings.

app/meson.build

@@ -141,6 +141,8 @@ install_man('scrcpy.1')
 
 ### TESTS
 
+# do not build tests in release (assertions would not be executed at all)
+if get_option('buildtype') == 'debug'
     tests = [
         ['test_buffer_util', [
             'tests/test_buffer_util.c'
@@ -178,3 +180,4 @@ foreach t : tests
                          c_args: ['-DSDL_MAIN_HANDLED'])
         test(t[0], exe)
     endforeach
+endif