unit-test: fix 'env default' invocation

"env default -f" doesn't work any more; replace it with
"env default -f -a". This avoids the following when running the ut
command:

do_ut_cmd: Testing commands
env - environment handling commands

Usage:
env default [-f] -a - [forcibly] reset default environment
...
Signed-off-by: NStephen Warren <swarren@nvidia.com>

test/command_ut.c

@@ -15,7 +15,7 @@ static const char test_cmd[] = "setenv list 1\n setenv list ${list}2; "
 static int do_ut_cmd(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 {
 	printf("%s: Testing commands\n", __func__);
-	run_command("env default -f", 0);
+	run_command("env default -f -a", 0);
 
 	/* run a single command */
 	run_command("setenv single 1", 0);