Enable or_fun_call lint to avoid unnecessary allocations (#7754)

### What

From the [lint
description](https://rust-lang.github.io/rust-clippy/master/index.html?search=or_fu#or_fun_call):

> The function will always be called. This is only bad if it allocates
or does some non-trivial amount of work.

But also:

> If the function has side-effects, not calling it will change the
semantic of the program, but you shouldn’t rely on that.
> 
> The lint also cannot figure out whether the function you call is
actually expensive to call or not.

Still worth it to keep our happy paths clean, imo.

crates/egui_kittest/src/snapshot.rs

@@ -264,7 +264,8 @@ impl Display for SnapshotError {
                 diff,
                 diff_path,
             } => {
-                let diff_path = std::path::absolute(diff_path).unwrap_or(diff_path.clone());
+                let diff_path =
+                    std::path::absolute(diff_path).unwrap_or_else(|_| diff_path.clone());
                 write!(
                     f,
                     "'{name}' Image did not match snapshot. Diff: {diff}, {}. {HOW_TO_UPDATE_SCREENSHOTS}",
@@ -272,7 +273,7 @@ impl Display for SnapshotError {
                 )
             }
             Self::OpenSnapshot { path, err } => {
-                let path = std::path::absolute(path).unwrap_or(path.clone());
+                let path = std::path::absolute(path).unwrap_or_else(|_| path.clone());
                 match err {
                     ImageError::IoError(io) => match io.kind() {
                         ErrorKind::NotFound => {
@@ -310,7 +311,7 @@ impl Display for SnapshotError {
                 )
             }
             Self::WriteSnapshot { path, err } => {
-                let path = std::path::absolute(path).unwrap_or(path.clone());
+                let path = std::path::absolute(path).unwrap_or_else(|_| path.clone());
                 write!(f, "Error writing snapshot: {err}\nAt: {}", path.display())
             }
             Self::RenderError { err } => {

crates/egui_kittest/src/wgpu.rs

@@ -51,7 +51,7 @@ pub fn default_wgpu_setup() -> egui_wgpu::WgpuSetup {
         adapters
             .first()
             .map(|a| (*a).clone())
-            .ok_or("No adapter found".to_owned())
+            .ok_or_else(|| "No adapter found".to_owned())
     }));
 
     egui_wgpu::WgpuSetup::CreateNew(setup)