niri-config: Add pretty-assertions to tests

The config parse test is pretty big and it's impossible to tell the
difference from the normal assert.

Cargo.lock

@@ -2216,6 +2216,7 @@ dependencies = [
  "knuffel",
  "miette",
  "niri-ipc",
+ "pretty_assertions",
  "regex",
  "smithay",
  "tracing",
@@ -2682,6 +2683,16 @@ version = "0.2.17"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5b40af805b3121feab8a3c29f04d8ad262fa8e0561883e7653e024ae4479e6de"
 
+[[package]]
+name = "pretty_assertions"
+version = "1.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "af7cee1a6c8a5b9208b3cb1061f10c0cb689087b3d8ce85fb9d2dd7a29b6ba66"
+dependencies = [
+ "diff",
+ "yansi",
+]
+
 [[package]]
 name = "proc-macro-crate"
 version = "1.3.1"
@@ -4375,6 +4386,12 @@ version = "0.2.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9d422e8e38ec76e2f06ee439ccc765e9c6a9638b9e7c9f2e8255e4d41e8bd852"
 
+[[package]]
+name = "yansi"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "09041cd90cf85f7f8b2df60c646f853b7f535ce68f85244eb6731cf89fa498ec"
+
 [[package]]
 name = "yansi-term"
 version = "0.1.2"