Revert "disable flaky test/incremental/add_decl"

This reverts commit d54fbc0123.

Since all incremental tests are flaky on Windows, this is reinstated and
all test-incremental tests will be skipped on Windows until the
flakiness is resolved.

Closes #26003

test/incremental/add_decl

@@ -0,0 +1,63 @@
+#target=x86_64-linux-selfhosted
+#target=x86_64-windows-selfhosted
+#target=x86_64-linux-cbe
+#target=x86_64-windows-cbe
+//#target=wasm32-wasi-selfhosted
+#update=initial version
+#file=main.zig
+const std = @import("std");
+pub fn main() !void {
+    try std.fs.File.stdout().writeAll(foo);
+}
+const foo = "good morning\n";
+#expect_stdout="good morning\n"
+
+#update=add new declaration
+#file=main.zig
+const std = @import("std");
+pub fn main() !void {
+    try std.fs.File.stdout().writeAll(foo);
+}
+const foo = "good morning\n";
+const bar = "good evening\n";
+#expect_stdout="good morning\n"
+
+#update=reference new declaration
+#file=main.zig
+const std = @import("std");
+pub fn main() !void {
+    try std.fs.File.stdout().writeAll(bar);
+}
+const foo = "good morning\n";
+const bar = "good evening\n";
+#expect_stdout="good evening\n"
+
+#update=reference missing declaration
+#file=main.zig
+const std = @import("std");
+pub fn main() !void {
+    try std.fs.File.stdout().writeAll(qux);
+}
+const foo = "good morning\n";
+const bar = "good evening\n";
+#expect_error=main.zig:3:39: error: use of undeclared identifier 'qux'
+
+#update=add missing declaration
+#file=main.zig
+const std = @import("std");
+pub fn main() !void {
+    try std.fs.File.stdout().writeAll(qux);
+}
+const foo = "good morning\n";
+const bar = "good evening\n";
+const qux = "good night\n";
+#expect_stdout="good night\n"
+
+#update=remove unused declarations
+#file=main.zig
+const std = @import("std");
+pub fn main() !void {
+    try std.fs.File.stdout().writeAll(qux);
+}
+const qux = "good night\n";
+#expect_stdout="good night\n"