lib: check kconfig file existence in tst_has_slow_kconfig

In the GitHub CI environment, or certain images used during testing,
the kconfig files might not be present. Without checking for the
existence of the kconfig path, the test attempts to read a non-existent
file, leading to errors or test failures.

  tst_kconfig.c:207: TBROK: Cannot parse kernel .config

Signed-off-by: Li Wang <[email protected]>

lib/tst_kconfig.c

@@ -655,6 +655,10 @@ static struct tst_kconfig_var slow_kconfigs[] = {
 int tst_has_slow_kconfig(void)
 {
 	unsigned int i;
+	char path_buf[1024];
+
+	if (!kconfig_path(path_buf, sizeof(path_buf)))
+		return 0;
 
 	tst_kconfig_read(slow_kconfigs, ARRAY_SIZE(slow_kconfigs));