vmm, build: Remove use of "credibility" from unit tests

This crate was used in the integration tests to allow the tests to
continue and clean up after a failure. This isn't necessary in the unit
tests and adds a large build dependency chain including an unmaintained
crate.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vmm/src/lib.rs

@@ -11,9 +11,6 @@ extern crate lazy_static;
 extern crate log;
 #[macro_use]
 extern crate serde_derive;
-#[cfg(test)]
-#[macro_use]
-extern crate credibility;
 
 use crate::api::{
     ApiError, ApiRequest, ApiResponse, ApiResponsePayload, VmInfo, VmReceiveMigrationData,