Fix some clippy issues

Found while working on https://github.com/rust-lang/rust/pull/77351;
these are just the ones that could be fixed automatically.

library/test/src/bench.rs

@@ -159,7 +159,7 @@ where
             return summ5;
         }
 
-        total_run = total_run + loop_run;
+        total_run += loop_run;
         // Longest we ever run for is 3s.
         if total_run > Duration::from_secs(3) {
             return summ5;

library/test/src/formatters/pretty.rs

@@ -139,7 +139,7 @@ impl<T: Write> PrettyFormatter<T> {
                 stdouts.push_str(&format!("---- {} stdout ----\n", f.name));
                 let output = String::from_utf8_lossy(stdout);
                 stdouts.push_str(&output);
-                stdouts.push_str("\n");
+                stdouts.push('\n');
             }
         }
         if !stdouts.is_empty() {

library/test/src/formatters/terse.rs

@@ -114,7 +114,7 @@ impl<T: Write> TerseFormatter<T> {
                 stdouts.push_str(&format!("---- {} stdout ----\n", f.name));
                 let output = String::from_utf8_lossy(stdout);
                 stdouts.push_str(&output);
-                stdouts.push_str("\n");
+                stdouts.push('\n');
             }
         }
         if !stdouts.is_empty() {
@@ -140,7 +140,7 @@ impl<T: Write> TerseFormatter<T> {
                 fail_out.push_str(&format!("---- {} stdout ----\n", f.name));
                 let output = String::from_utf8_lossy(stdout);
                 fail_out.push_str(&output);
-                fail_out.push_str("\n");
+                fail_out.push('\n');
             }
         }
         if !fail_out.is_empty() {

library/test/src/lib.rs

@@ -237,11 +237,9 @@ where
     let event = TestEvent::TeFiltered(filtered_descs);
     notify_about_test_event(event)?;
 
-    let (filtered_tests, filtered_benchs): (Vec<_>, _) =
-        filtered_tests.into_iter().partition(|e| match e.testfn {
-            StaticTestFn(_) | DynTestFn(_) => true,
-            _ => false,
-        });
+    let (filtered_tests, filtered_benchs): (Vec<_>, _) = filtered_tests
+        .into_iter()
+        .partition(|e| matches!(e.testfn, StaticTestFn(_) | DynTestFn(_)));
 
     let concurrency = opts.test_threads.unwrap_or_else(get_concurrency);
 

library/test/src/stats.rs

@@ -199,7 +199,7 @@ impl Stats for [f64] {
             let mut v: f64 = 0.0;
             for s in self {
                 let x = *s - mean;
-                v = v + x * x;
+                v += x * x;
             }
             // N.B., this is _supposed to be_ len-1, not len. If you
             // change it back to len, you will be calculating a