behavior: add test coverage for slice and array-related issues

Closes ziglang#10684
Closes ziglang#6905
Closes ziglang#8646

test/behavior.zig

@@ -65,14 +65,17 @@ test {
     _ = @import("behavior/bugs/6456.zig");
     _ = @import("behavior/bugs/6781.zig");
     _ = @import("behavior/bugs/6850.zig");
+    _ = @import("behavior/bugs/6905.zig");
     _ = @import("behavior/bugs/7003.zig");
     _ = @import("behavior/bugs/7047.zig");
     _ = @import("behavior/bugs/7187.zig");
     _ = @import("behavior/bugs/7325.zig");
+    _ = @import("behavior/bugs/8646.zig");
     _ = @import("behavior/bugs/9584.zig");
     _ = @import("behavior/bugs/10138.zig");
     _ = @import("behavior/bugs/10147.zig");
     _ = @import("behavior/bugs/10970.zig");
+    _ = @import("behavior/bugs/10684.zig");
     _ = @import("behavior/bugs/11046.zig");
     _ = @import("behavior/bugs/11100.zig");
     _ = @import("behavior/bugs/11139.zig");

test/behavior/bugs/10684.zig

@@ -0,0 +1,12 @@
+const std = @import("std");
+const expectEqualStrings = std.testing.expectEqualStrings;
+
+test "slicing slices" {
+    const foo = "1234";
+    const bar = foo[0..4];
+    try expectEqualStrings("1234", bar);
+    try expectEqualStrings("2", bar[1..2]);
+    try expectEqualStrings("3", bar[2..3]);
+    try expectEqualStrings("4", bar[3..4]);
+    try expectEqualStrings("34", bar[2..4]);
+}

test/behavior/bugs/6905.zig

@@ -0,0 +1,16 @@
+const expect = @import("std").testing.expect;
+
+test "sentinel-terminated 0-length slices" {
+    var u32s: [4]u32 = [_]u32{0, 1, 2, 3};
+
+    var index: u8 = 2;
+    var slice = u32s[index..index:2];
+    var array_ptr = u32s[2..2:2];
+    const comptime_known_array_value = u32s[2..2:2].*;
+    var runtime_array_value = u32s[2..2:2].*;
+
+    try expect(slice[0] == 2);
+    try expect(array_ptr[0] == 2);
+    try expect(comptime_known_array_value[0] == 2);
+    try expect(runtime_array_value[0] == 2);
+}

test/behavior/bugs/8646.zig

@@ -0,0 +1,11 @@
+const std = @import("std");
+
+const array = [_][]const []const u8{
+  &.{ "hello" },
+  &.{ "world", "hello" },
+};
+
+test {
+    try std.testing.expect(array[0].len == 1);
+    try std.testing.expectEqualStrings("hello", array[0][0]);
+}