carddav: Try out some tests with insta

Cargo.lock

@@ -1789,6 +1789,17 @@ version = "0.1.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c8fae54786f62fb2918dcfae3d568594e50eb9b5c25bf04371af6fe7516452fb"
 
+[[package]]
+name = "insta"
+version = "1.44.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b5c943d4415edd8153251b6f197de5eb1640e56d84e8d9159bea190421c73698"
+dependencies = [
+ "console",
+ "once_cell",
+ "similar",
+]
+
 [[package]]
 name = "ipnet"
 version = "2.11.0"
@@ -3142,6 +3153,7 @@ dependencies = [
  "futures-util",
  "http",
  "ical",
+ "insta",
  "percent-encoding",
  "quick-xml",
  "rustical_dav",