mdx blocks that use z3 are only enabled when the only installed solver is z3

[filipeom]

I think I did this because there were errors when running dune runtest.

smtml/dune

Lines 1 to 8 in 131c8ca

	(mdx
	(enabled_if
	(and
	%{lib-available:z3}
	(not %{lib-available:alt-ergo-lib})
	(not %{lib-available:bitwuzla-cxx})
	(not %{lib-available:colibri2.core})
	(not %{lib-available:cvc5}))))

But we should try to make it:

(mdx
  (enabled_if %{lib-available:z3}))