Update Rust crate httptest to 0.16.1 (oxidecomputer#1410)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [httptest](https://togithub.com/ggriffiniii/httptest) |
workspace.dependencies | minor | `0.15.5` -> `0.16.1` |

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### Release Notes

<details>
<summary>ggriffiniii/httptest (httptest)</summary>

###
[`v0.16.0`](https://togithub.com/ggriffiniii/httptest/compare/v0.15.5...v0.16.0)

[Compare
Source](https://togithub.com/ggriffiniii/httptest/compare/v0.15.5...v0.16.0)

</details>

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Cargo.toml

@@ -52,7 +52,7 @@ futures = "0.3"
 futures-core = "0.3"
 hex = "0.4"
 http = "0.2.12"
-httptest = "0.15.5"
+httptest = "0.16.1"
 human_bytes = "0.4.3"
 humantime = "2.1.0"
 hyper = { version = "0.14", features = [ "full" ] }