test: Fix bootm_test_subst_var() running independently

This test relies on the silent_linux env variable being set. Add this
to the code so it can run without relying on other bootm tests having been
run first.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/bootm.c

@@ -208,7 +208,8 @@ BOOTM_TEST(bootm_test_silent_var, 0);
 /* Test substitution processing in the bootargs variable */
 static int bootm_test_subst_var(struct unit_test_state *uts)
 {
-	env_set("bootargs", NULL);
+	ut_assertok(env_set("silent_linux", "yes"));
+	ut_assertok(env_set("bootargs", NULL));
 	ut_assertok(bootm_process_cmdline_env(BOOTM_CL_SILENT));
 	ut_asserteq_str("console=ttynull", env_get("bootargs"));