user: add test-cases for maxID (math.MaxInt32)

[thaJeztah]

• relates to oci: return explicit error for out-of-range USER values containerd/containerd#13446
• relates to containerd/containerd@de1341c

[vvoland]

This only tests the case where gid/uid still falls within int .

If it's larger, it will still resolve to root because it will be treated as an username and still resolved.

So:

9223372036854775808:x:0:0:root:/root:/bin/sh

• USER 9223372036854775808

Means that the runtime will actually resolve it to root.

This is "correct" behavior, but its confusing to treat a numeric value as an username.

[thaJeztah]

@vvoland - trying to see where that codepath would be; testing GetExecUser with 2147483648 (math.MaxInt32 + 1) produces an error; or is it a different codepath you're looking at?

[vvoland]

On 64 bit strconv.Atoi will handle MaxInt32 + 1 without an error