test/py: Add tests for the SquashFS commands

Add Python scripts to test 'ls' and 'load' commands. The scripts
generate a SquashFS image and clean the directory after the assertions,
or if an exception is raised.

Signed-off-by: Joao Marcos Costa <joaomarcos.costa@bootlin.com>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 65cfc85..8309ce6 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -89,6 +89,7 @@
 CONFIG_CMD_TPM_TEST=y
 CONFIG_CMD_BTRFS=y
 CONFIG_CMD_CBFS=y
+CONFIG_CMD_SQUASHFS=y
 CONFIG_CMD_CRAMFS=y
 CONFIG_CMD_EXT4_WRITE=y
 CONFIG_CMD_MTDPARTS=y