Add a test that should fail but doesn't

[schneems]

Edit: Here's the code that produces the original error heroku/buildpacks-ruby#369

There's a bug in the macros where this code is missing a semicolon:

        // Link B => C, and the rest
        $crate::try_migrate_link!($b, $($rest),*)

It should be triggered by the code in this test but it's not.

It mimics this code from the Ruby buildpack:

try_migrate_deserializer_chain!(
    chain: [MetadataV1, MetadataV2, MetadataV3],
    error: MetadataMigrateError,
    deserializer: toml::Deserializer::new,
);

Which triggers this error (backtrace courtesy of nightly) RUSTFLAGS="-Zmacro-backtrace" cargo build

$ RUSTFLAGS="-Zmacro-backtrace" cargo build 
   Compiling heroku-ruby-buildpack v0.0.0 (/Users/rschneeman/Documents/projects/work/buildpacks-ruby/buildpacks/ruby)
error: macros that expand to items must be delimited with braces or followed by a semicolon
   --> /Users/rschneeman/.cargo/registry/src/index.crates.io-6f17d22bba15001f/magic_migrate-0.2.0/src/lib.rs:437:34
    |
419 |   macro_rules! try_migrate_link {
    |   ----------------------------- in this expansion of `$crate::try_migrate_link!` (#3)
...
437 |           $crate::try_migrate_link!($b, $($rest),*)
    |                                    ^^^^^^^^^^^^^^^^
...
714 |   macro_rules! try_migrate_deserializer_chain {
    |   -------------------------------------------
    |   |
    |   in this expansion of `try_migrate_deserializer_chain!` (#1)
    |   in this expansion of `$crate::try_migrate_deserializer_chain!` (#2)
...
737 |           $crate::try_migrate_link!($a, $($rest),+);
    |           ----------------------------------------- in this macro invocation (#3)
...
764 |           $crate::try_migrate_deserializer_chain!(error: $err, deserializer: $deser, chain: [$a, $($rest),+]);
    |           --------------------------------------------------------------------------------------------------- in this macro invocation (#2)
    |
   ::: buildpacks/ruby/src/layers/bundle_install_layer.rs:120:1
    |
120 | / try_migrate_deserializer_chain!(
121 | |     chain: [MetadataV1, MetadataV2, MetadataV3],
122 | |     error: MetadataMigrateError,
123 | |     deserializer: toml::Deserializer::new,
124 | | );
    | |_- in this macro invocation (#1)

However, the current test compiles and runs just fine and I don't know why.