Add regression test of issue 186
Add regression test of issue 186

Currently fails to build without the fix:

    error: future cannot be shared between threads safely
      --> tests/test_macros.rs:64:5
       |
    64 |     require_send_sync(async {
       |     ^^^^^^^^^^^^^^^^^ future created by async block is not `Sync`
       |
       = help: within `impl Future`, the trait `Sync` is not implemented for `Cell<&str>`
    note: future is not `Sync` as this value is used across an await
      --> tests/test_macros.rs:65:9
       |
    65 |         future::ready(anyhow!(message(Cell::new("...")))).await;
       |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ first, await occurs here, with `Cell::new("...")` maybe used later...
    note: `Cell::new("...")` is later dropped here
      --> tests/test_macros.rs:65:64
       |
    65 |         future::ready(anyhow!(message(Cell::new("...")))).await;
       |                                       ----------------         ^
       |                                       |
       |                                       has type `Cell<&str>` which is not `Sync`
    note: required by a bound in `require_send_sync`
      --> tests/test_macros.rs:50:41
       |
    50 |     fn require_send_sync(_: impl Send + Sync) {}
       |                                         ^^^^ required by this bound in `require_send_sync`

dtolnay/anyhow