Fix FTBFS of test code if assert_cmd crate is updated to more recent versions

[mulkieran]

There's already a bunch of issues about this problem:

• Cannot use cargo_bin! without triggering deprecation warning for cargo_bin assert-rs/assert_cmd#258
• cargo::cargo_bin! vs cargo::cargo_bin_cmd! assert-rs/assert_cmd#263
• cargo_bin!() can’t take non-literals binary names assert-rs/assert_cmd#268

[mulkieran]

Not urgent, cause we're pinned to a lower version.