tests: add insta for snapshot testing

Insta makes writing new tests or performing changes which update the
expected value(s) easier. The new reference data can be reviewed
and inserted/updated automatically by `cargo insta review` (after
`cargo install cargo-insta`, but that tool is optional).

The snapshots are stored inline using the `@""` syntax and not in
separate files because the delta test output is small and designed to
be human readable.

See https://insta.rs/#hello-snapshot-testing
and https://docs.rs/insta/1.39.0/insta/

Cargo.toml

@@ -67,5 +67,14 @@ version = "0.29.0"
 default-features = false
 features = []
 
+[dev-dependencies]
+insta = { version = "1.*", features = ["colors"] }
+
 [profile.test]
 opt-level = 2
+
+[profile.dev.package.insta]
+opt-level = 3
+
+[profile.dev.package.similar]
+opt-level = 3

src/features/side_by_side.rs

@@ -594,16 +594,19 @@ pub mod tests {
     use crate::features::line_numbers::tests::*;
     use crate::options::theme;
     use crate::tests::integration_test_utils::{make_config_from_args, run_delta, DeltaTest};
+    use insta::assert_snapshot;
 
     #[test]
-    fn test_two_minus_lines() {
-        DeltaTest::with_args(&["--side-by-side", "--width", "40"])
+    fn test_two_fitting_minus_lines() {
+        // rustfmt ignores the assert macro arguments, so do the setup outside
+        let result = DeltaTest::with_args(&["--side-by-side", "--width", "40"])
             .with_input(TWO_MINUS_LINES_DIFF)
-            .expect_after_header(
-                r#"
-                │  1 │a = 1         │    │
-                │  2 │b = 23456     │    │"#,
-            );
+            .skip_header();
+        assert_snapshot!(result, @r###"
+        │  1 │a = 1         │    │
+        │  2 │b = 23456     │    │
+        "###
+        );
     }
 
     #[test]

src/tests/integration_test_utils.rs

@@ -6,7 +6,7 @@ use std::io::{BufReader, Write};
 use std::path::Path;
 
 use bytelines::ByteLines;
-use itertools;
+use itertools::Itertools;
 
 use crate::ansi;
 use crate::cli;
@@ -264,6 +264,10 @@ impl DeltaTestOutput {
         self.expect_after_skip(crate::config::HEADER_LEN, expected)
     }
 
+    pub fn skip_header(self) -> String {
+        self.output.lines().skip(config::HEADER_LEN).join("\n")
+    }
+
     pub fn expect_contains(self, expected: &str) -> Self {
         assert!(
             self.output.contains(expected),