Rollup merge of #136160 - ShE3py:should-panic-backticks, r=thomcc

Remove backticks from `ShouldPanic::YesWithMessage`'s `TrFailedMsg`

More legible imo
```rs
#[test]
#[should_panic = "love"]
fn foo() {
    assert!(1 == 2);
}
```
Before:
```
note: panic did not contain expected string
      panic message: `"assertion failed: 1 == 2"`,
 expected substring: `"love"`
```
After:
```
note: panic did not contain expected string
      panic message: "assertion failed: 1 == 2"
 expected substring: "love"
```
Also removed the comma as `assert_eq!` / `assert_ne!` don't use one.

``@rustbot`` label +A-libtest

Author: matthiaskrgr