Don't rely on mtime to test changes

tests/testsuite/build_script.rs

@@ -3471,7 +3471,7 @@ fn rebuild_only_on_explicit_paths() {
 
     // random other files do not affect freshness
     println!("run baz");
-    p.change_file("baz", "");
+    p.change_file("baz", "// modified");
     p.cargo("build -v")
         .with_stderr(
             "\
@@ -3483,7 +3483,7 @@ fn rebuild_only_on_explicit_paths() {
 
     // but changing dependent files does
     println!("run foo change");
-    p.change_file("foo", "");
+    p.change_file("foo", "// modified");
     p.cargo("build -v")
         .with_stderr(
             "\

tests/testsuite/build_script_env.rs

@@ -96,7 +96,7 @@ fn rerun_if_env_or_file_changes() {
         .with_stderr("[FINISHED] [..]")
         .run();
     sleep_ms(1000);
-    p.change_file("foo", "");
+    p.change_file("foo", "// modified");
     p.cargo("check")
         .env("FOO", "bar")
         .with_stderr(

tests/testsuite/freshness.rs

@@ -1522,7 +1522,7 @@ fn bust_patched_dep() {
         sleep_ms(1000);
     }
 
-    p.change_file("reg1new/src/lib.rs", "");
+    p.change_file("reg1new/src/lib.rs", "// modified");
     if is_coarse_mtime() {
         sleep_ms(1000);
     }