Discard auto_viz_demo's trace output in its CTest test

auto_viz's pipelines are built with trace_all, but auto_viz_demo.cpp
never sets HL_TRACE_FILE, so Halide's default trace handler falls back
to formatting every load/store/produce/consume event as plain text via
halide_print. For even the small test image that's tens of millions of
lines (multiple GB) of stdout. Direct execution is merely slow, but a
test runner that captures/streams that output (e.g. `ctest -V`, as the
buildbot uses) can burn through the entire test timeout on it.

Point HL_TRACE_FILE at the null device for the auto_viz_demo test so
it exercises the same binary trace-writing path (still a meaningful
check that trace_all pipelines run) while discarding the output almost
instantly.

Author: alexreinking

apps/auto_viz/CMakeLists.txt

@@ -40,6 +40,23 @@ add_executable(auto_viz_demo auto_viz_demo.cpp)
 target_link_libraries(auto_viz_demo PRIVATE Halide::ImageIO ${variants})
 
 # Test that the app actually works!
+#
+# The libraries above are built with trace_all, and auto_viz_demo.cpp never
+# installs a custom trace handler or points HL_TRACE_FILE anywhere, so by
+# default every load/store/produce/consume event is formatted as plain text
+# and printed via halide_print. For even this small test image that's tens of
+# millions of lines (multiple GB) of stdout, which is merely slow to produce
+# directly but can consume a CI test run's entire timeout once a test runner
+# is capturing/streaming that output (e.g. `ctest -V`). Route the trace to
+# the null device instead: this exercises the exact same tracing code path
+# (so the test remains a meaningful check that trace_all pipelines still run)
+# while discarding the output near-instantly.
+if (WIN32)
+    set(_auto_viz_trace_sink "NUL")
+else ()
+    set(_auto_viz_trace_sink "/dev/null")
+endif ()
+
 set(IMAGE ${CMAKE_CURRENT_LIST_DIR}/../images/rgb_small.png)
 if (EXISTS ${IMAGE})
     configure_file(${IMAGE} rgb_small.png COPYONLY)
@@ -48,6 +65,7 @@ if (EXISTS ${IMAGE})
         auto_viz_demo
         PROPERTIES
         LABELS auto_viz
+        ENVIRONMENT "HL_TRACE_FILE=${_auto_viz_trace_sink}"
         SKIP_REGULAR_EXPRESSION "\\[SKIP\\]"
     )
 endif ()