std.EnumArray: edit return of iterator.next() (Reloaded)

[poypoyan]

Fixes: #22506

(No changes from previous PR; just moved to a new branch so that master is sync for new contribution.)

The main fix is to replace return Entry{ with just return .{ in iterator.next() of EnumArray. The prior should actually work, but I think there is a deeper bug here: I see 3 "variants" of issue #19985:

const E = @Type(.{ .Enum = .{
    .tag_type = u0,
    .is_exhaustive = true,
    .fields = &.{.{ .name = "foo", .value = 0 }},
    .decls = &.{},
} });

const s: struct { E } = .{.foo};   // 1) original of #19955. compiles.
const s = struct { k: E }{ .k = .foo };   // 2) minimal replication of the current issue. does not compile!
const s: struct { k: E } = .{ .k = .foo };   // 3) minimal replication of my fix. compiles.

Thus my change in test/behavior/type.zig is to add variant 3) alongside 1). I am not knowledgeable in how compiler works so I cannot investigate further on why 2) doesn't work.

Other changes:

• Moved EnumSet tests below EnumSet function.
• Add test for the current issue below EnumArray function.