CI: Reduce parallelism of render tests on macOS in CI only.

We've been getting frequent GPU out-of-memory errors, and I suspect
they're due to a memory leak but it might be just limited resources.
So, let's see what this does.

test-renderers/src/harness.rs

@@ -210,6 +210,7 @@ pub async fn harness_main<Factory, Ff>(
     renderer_id: RendererId,
     test_suite: fn(&mut TestCaseCollector<'_>),
     factory_factory: Ff, // TODO: better name
+    max_parallelism: Option<usize>,
 ) -> HarnessResult
 where
     Factory: RendererFactory + 'static,
@@ -335,7 +336,7 @@ where
             }
             .boxed()
         })
-        .buffer_unordered(4);
+        .buffer_unordered(max_parallelism.unwrap_or(4));
 
     let mut per_test_output = BTreeMap::new();
     let mut count_passed = 0;

test-renderers/tests/ray-render.rs

@@ -16,6 +16,7 @@ async fn main() -> test_renderers::HarnessResult {
         RendererId::Raytracer,
         test_renderers::test_cases::all_tests,
         || std::future::ready(RtFactory),
+        None,
     )
     .await
 }

test-renderers/tests/wgpu-render.rs

@@ -23,11 +23,20 @@ async fn main() -> test_renderers::HarnessResult {
         return ExitCode::SUCCESS;
     };
 
+    let parallelism = if option_env!("CI").is_some() && cfg!(target_os = "macos") {
+        // Attempted workaround for out-of-memory on macOS CI. If this doesn't help then
+        // the problem must be a leak, not concurrency.
+        Some(1)
+    } else {
+        None
+    };
+
     test_renderers::harness_main(
         test_renderers::HarnessArgs::parse(),
         RendererId::Wgpu,
         test_renderers::test_cases::all_tests,
         get_factory,
+        parallelism,
     )
     .await
 }