This is the gold standard for Spectrum diagnostics. It is often what people mean when they ask for a "test program."
: A utility that converts a plain text file containing BASIC code into a .tap file that can be loaded into an emulator or real hardware [9]. zx spectrum test program link
Chaining technique details:
You can test your code or these diagnostic files immediately using these emulators: This is the gold standard for Spectrum diagnostics
If the link above is offline, check the Sinclair ZX World forums or Planet Sinclair’s tools section for updated diagnostic files. zx spectrum test program link