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

@@ -141,7 +141,9 @@ install_man('scrcpy.1')
 
 ### TESTS
 
-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'
         ]],
@@ -169,12 +171,13 @@ tests = [
             'tests/test_strutil.c',
             'src/util/str_util.c',
         ]],
-]
+    ]
 
-foreach t : tests
+    foreach t : tests
         exe = executable(t[0], t[1],
                          include_directories: src_dir,
                          dependencies: dependencies,
                          c_args: ['-DSDL_MAIN_HANDLED'])
         test(t[0], exe)
-endforeach
+    endforeach
+endif