test: cmd: exit: Add unit test for exit and partly run commands

Add a test which validates that exit from environment script works as
expected, including return value propagation and clipping to positive
integers.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Marek Vasut <marex@denx.de>

test/cmd/Makefile

@@ -8,7 +8,7 @@ endif
 ifdef CONFIG_CONSOLE_RECORD
 obj-$(CONFIG_CMD_PAUSE) += test_pause.o
 endif
-obj-y += mem.o
+obj-y += exit.o mem.o
 obj-$(CONFIG_CMD_ADDRMAP) += addrmap.o
 obj-$(CONFIG_CMD_FDT) += fdt.o
 obj-$(CONFIG_CONSOLE_TRUETYPE) += font.o