Update `std.testing.expectEqual` and friends to use peer type resolution

This commit changes the type of the second parameter to `anytype`, which should make it easier to pass literals to these functions. This change shouldn't *silently* break existing code (the assertions themselves should retain the same behavior as before) but it may result in some new compile errors when struct/union/array literals or builtins like `@bitCast` are used for the second argument. These compile errors can be fixed by explicitly coercing these expressions to the correct type using `@as`.