[nexus-config] use pretty_assertions in tests (#7798)

Currently, assertion failures from the `nexus-config` tests are pretty
hard to interpret. When we assert a parsed config matches the expected
config, we use `assert_eq!`, which uses the default `fmt::Debug`
formatter. As the config types are quite complex, this spits out a huge
amount of text, and it's all on one line. the differences between the
actual and expected onfig are pretty hard to find. For example, see
[this failure on Buildomat][1].

This commit changes these tests to use the `pretty_assertions` crate's
`assert_eq!` macro, which formats the difference between the actual and
expected values as a diff, highlighting what didn't match. I also
considered using `daft` to diff these values, but `pretty_assertions` is
already in our dep tree, and didn't require any additional code outside
of the tests (where `daft` would require deriving stuff for the config
types). For this use case, the simple textual diff seems sufficient.

[1]:
https://buildomat.eng.oxide.computer/wg/0/details/01JP8BHQ5Z41DPB6GEK4D2K3N9/PhIpT14ekjFiDaHBPM5hDzWVhYr53VTLUXpaAAs6veI1rUJh/01JP8BJ6EZH8RRE1V27DGK7DDG#S6106

Author: hawkw

Cargo.lock

@@ -24,3 +24,4 @@ uuid.workspace = true
 expectorate.workspace = true
 libc.workspace = true
 serde_json.workspace = true
+pretty_assertions.workspace = true

nexus-config/Cargo.toml

@@ -830,6 +830,7 @@ mod test {
     use dropshot::ConfigLogging;
     use dropshot::ConfigLoggingIfExists;
     use dropshot::ConfigLoggingLevel;
+    use pretty_assertions::assert_eq;
     use std::collections::HashMap;
     use std::fs;
     use std::net::{Ipv6Addr, SocketAddr, SocketAddrV6};