Improve assertions in src/tests.rs (#10449)

# Description
Mention the expected substring in:
- `fn run_test_contains()`
- `fn test_ide_contains()`
- `fn fail_test()`


# Developer-Facing Changes
You can read the test failure to figure out what is going on, don't
necessarily have to look up the test

New look of failure
```
failures:

---- tests::test_math::broken_math stdout ----
stdout:
stderr: Error: nu::parser::incomplete_math_expression

  × Incomplete math expression.
   ╭─[/tmp/.tmpz0V8nC:1:1]
 1 │ 3 +
   ·   ┬
   ·   ╰── incomplete math expression
   ╰────


Expected error to contain: Incomplete universe
thread 'tests::test_math::broken_math' panicked at 'assertion failed: !stderr.is_empty() && stderr.contains(expected)', src/tests.rs:181:5
```

src/tests.rs

@@ -116,6 +116,7 @@ pub fn run_test_contains(input: &str, expected: &str) -> TestResult {
     println!("stdout: {stdout}");
     println!("stderr: {stderr}");
 
+    println!("Expected output to contain: {expected}");
     assert!(output.status.success());
 
     assert!(stdout.contains(expected));
@@ -144,6 +145,7 @@ pub fn test_ide_contains(input: &str, ide_commands: &[&str], expected: &str) ->
 
     println!("stdout: {stdout}");
     println!("stderr: {stderr}");
+    println!("Expected output to contain: {expected}");
 
     assert!(output.status.success());
 
@@ -174,6 +176,7 @@ pub fn fail_test(input: &str, expected: &str) -> TestResult {
 
     println!("stdout: {stdout}");
     println!("stderr: {stderr}");
+    println!("Expected error to contain: {expected}");
 
     assert!(!stderr.is_empty() && stderr.contains(expected));