Then YHWH said to Moses, "Behold, I am about to rain bread from heaven for you, and the people shall go out and gather a day's portion every day, that I may test them, whether they will walk in my law or not."
Exodus 16:4

God already knew who they were and his promise was to Abraham, not them, so it couldn't have been a test of their worthiness. Like so many of God's tests, this one was for Israel's benefit. They needed to see their own character and whether or not they would be able to keep a simple instruction.

The double-portion on the sixth day wasn't meant to imply that God would always provide material blessings to help us obey, but a further demonstration that we would not keep the Sabbath no matter how easy God made it.