cargo test failed at test test_demo_app .

**Describe the bug**
```
test test_demo_app ... FAILED

failures:

---- test_demo_app stdout ----
thread 'test_demo_app' panicked at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/ptr/mod.rs:542:1:
Snapshot errors:
Error decoding snapshot: Decoding(DecodingError { format: Exact(Png), underlying: Some(Format(FormatError { inner: InvalidSignature })) })
At: "/home/john/projects/egui/crates/egui_demo_app/tests/snapshots/easymarkeditor.png". Make sure git-lfs is setup correctly. Read the instructions here: https://github.com/emilk/egui/blob/master/CONTRIBUTING.md#making-a-pr
Error decoding snapshot: Decoding(DecodingError { format: Exact(Png), underlying: Some(Format(FormatError { inner: InvalidSignature })) })
At: "/home/john/projects/egui/crates/egui_demo_app/tests/snapshots/clock.png". Make sure git-lfs is setup correctly. Read the instructions here: https://github.com/emilk/egui/blob/master/CONTRIBUTING.md#making-a-pr

stack backtrace:
0: rust_begin_unwind
at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/panicking.rs:665:5
1: core::panicking::panic_fmt
at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/panicking.rs:74:14
2: core::panicking::panic_display
at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/panicking.rs:264:5
3: ::drop::panic_cold_display
at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/panic.rs:101:13
4: ::drop
at /home/john/projects/egui/crates/egui_kittest/src/snapshot.rs:591:13
5: core::ptr::drop_in_place
at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/ptr/mod.rs:542:1
6: test_demo_app::test_demo_app
at ./tests/test_demo_app.rs:74:1
7: test_demo_app::test_demo_app::{{closure}}
at ./tests/test_demo_app.rs:8:19
8: core::ops::function::FnOnce::call_once
at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/ops/function.rs:250:5
9: core::ops::function::FnOnce::call_once
at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

failures:
test_demo_app

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.70s
```

**To Reproduce**
Steps to reproduce the behavior:
1. Check out current master of egui
2. cargo test

**Expected behavior**

**Screenshots**

**Desktop (please complete the following information):**
- OS: Ubuntu 22.04.5 LTS

**Additional context**

Start with crates/egui_demo_app/tests/test_demo_app.rs and the failing snapshots at crates/egui_demo_app/tests/snapshots/easymarkeditor.png and clock.png. Check the Git LFS setup using the CONTRIBUTING.md instructions, then rerun cargo test; done means test_demo_app passes and both snapshots decode successfully.

Written by the indexing model from the issue text.