Render tests: two moved raster tests no longer run alongside the tests that add fonts

The key of a raster made for content that shows a symbol includes the
font generation, and since c348b384 (#1723) the key for content
painted with a pattern includes it too. FontDb::add_font increases
that generation, and the six tests in tests_fontchange.rs call it. Two
tests in fx.rs draw such content and count the rasters moved between
frames: moved_rasters_need_the_same_content_and_effects and
a_moved_group_draws_the_shadow_of_the_instance_art_in_it_while_interacting.
Neither held fonts_steady(). When a font test added a font between
their frames, the renderer made the raster again and the test counted
0 rasters moved instead of 1. Under CPU load on Windows, with the font
tests in the same process, the first test failed in 12 of 100 runs and
the second failed in 36 of 70.

Both tests now hold fonts_steady().
translucent_blended_and_shadowed_type_redraws_when_its_font_becomes_available
added its font without the font lock that fonts_steady() holds shared,
and it now takes adding_fonts(), as the other five font tests do. With
the change, the first test passed in all 200 runs and the second
passed in all 120 runs under the same load.

crates/render/src/fx.rs

@@ -1514,6 +1514,7 @@ mod tests {
     use vectorcraft_geom::{Affine, Rect, shapes};
 
     use super::{Drawn, PIXEL_BUDGET, filter_region};
+    use crate::tests_fontchange::fonts_steady;
     use crate::{RenderOptions, Rendered, Renderer};
 
     /// #1706: the raster a shadow or glow is filtered in stays within the pixel budget however
@@ -1886,6 +1887,7 @@ mod tests {
     /// shadow is made from the instance's art, which the renderer measures the group's content by.
     #[test]
     fn a_moved_group_draws_the_shadow_of_the_instance_art_in_it_while_interacting() {
+        let _fonts = fonts_steady();
         let mut base = Document::new(100.0, 100.0);
         let orange = Appearance::basic(Paint::solid(Color::rgb(0.9, 0.4, 0.1)), Paint::None, 0.0);
         let art = Node::path(base.alloc_id(), shapes::rectangle(Rect::new(100.0, 100.0, 130.0, 126.0)), orange);
@@ -1929,6 +1931,7 @@ mod tests {
     /// again. A shadow keeps its raster under a new fill color.
     #[test]
     fn moved_rasters_need_the_same_content_and_effects() {
+        let _fonts = fonts_steady();
         let (view, still) = (Affine::scale(2.0), RenderOptions { background: Some([255, 255, 255, 255]), ..Default::default() });
         let live = RenderOptions { interacting: true, ..still.clone() };
         // `before` drawn, then `after` while interacting: the rasters drawn moved, and whether the

crates/render/src/tests_fontchange.rs

@@ -87,6 +87,7 @@ fn type_with_raster_effects_redraws_when_its_font_becomes_available() {
 /// available: the glyph geometry kept for it is laid out again.
 #[test]
 fn translucent_blended_and_shadowed_type_redraws_when_its_font_becomes_available() {
+    let _adding = adding_fonts();
     const FAMILY: &str = "Blended Serif4";
     let mut d = Document::new(300.0, 150.0);
     let l = d.layers[0].id;