Tests: the Alt-drag tests no longer fail when their documents are made in different seconds

file.new stamps metadata.created from the clock in whole seconds. Four
Alt-drag tests in tests_xform.rs compare the document under test with
one that a second session builds, and both sessions start from
session()'s file.new. When the clock reached a new second between the
two calls, the documents differed in created, and the test failed.
With a temporary change that made each clock read on a thread at least
one second later than the one before, all four failed.

session() in tests_xform.rs and persp_session() in
tests_persp_select.rs now pass "created": null to file.new, as #1894
did for doc_with_rect() in tests_ids_without_selection.rs. Two new
tests in #2067 compare two documents built on persp_session().

crates/engine/src/tests_persp_select.rs

@@ -11,7 +11,8 @@ use super::*;
 /// An 800 × 600 document with a two-point grid and a rectangle attached to the right plane.
 pub(crate) fn persp_session() -> (Session, NodeId) {
     let mut s = Session::new();
-    s.execute("file.new", &json!({"width": 800, "height": 600})).unwrap();
+    // Separately created fixtures must compare equal across clock ticks.
+    s.execute("file.new", &json!({"width": 800, "height": 600, "created": null})).unwrap();
     s.execute("perspective.grid.preset", &json!({"kind": 2})).unwrap();
     // Snap to Grid (on by default) would land moved edges on gridlines: these tests measure exact moves.
     s.execute("perspective.grid.snap", &json!({"on": false})).unwrap();

crates/engine/src/tests_xform.rs

@@ -9,7 +9,8 @@ use super::*;
 
 fn session() -> Session {
     let mut s = Session::new();
-    s.execute("file.new", &json!({"width": 800, "height": 600})).unwrap();
+    // Separately created fixtures must compare equal across clock ticks.
+    s.execute("file.new", &json!({"width": 800, "height": 600, "created": null})).unwrap();
     s
 }