Reduce the number of gtest-parallel workers when running Impeller tests on Mac Minis used by CI (#189813)

https://github.com/flutter/flutter/pull/188889 started using the
Impeller rendering backends in playground tests run by the
impeller_unittests harness.

Since then, runs of OpenGLESSDF tests on macOS CI have been flaky. These
flakes show up as gtest-parallel child processes that fail with exit
code -5 (SIGTRAP).

The affected CI machines are M1 Mac Minis (hardware model "Macmini9,1")
This is an experiment to see if limiting the number of test cases run in
parallel on these machines reduces the crashes.

See https://github.com/flutter/flutter/issues/189748

engine/src/flutter/testing/run_tests.py

diff --git a/engine/src/flutter/testing/run_tests.py b/engine/src/flutter/testing/run_tests.py
index 32dd2ba..df89a48 100755
--- a/engine/src/flutter/testing/run_tests.py
+++ b/engine/src/flutter/testing/run_tests.py
@@ -164,6 +164,12 @@
   return aarm64
 
 
+def mac_hardware_model() -> str:
+  assert is_mac()
+  output = subprocess.check_output(['sysctl', '-n', 'hw.model'])
+  return output.decode('utf-8').strip()
+
+
 def is_linux() -> bool:
   return sys_platform.startswith('linux')
 
@@ -568,7 +574,13 @@
       )
     extra_env = metal_validation_env()
     extra_env.update(vulkan_validation_env(build_dir))
-    mac_impeller_unittests_flags = repeat_flags + [
+    if mac_hardware_model() == 'Macmini9,1':
+      # For the Mac Minis used on CI, limit the number of Impeller test cases run in parallel
+      # in order to reduce the risk of resource exhaustion errors.
+      workers_flag = ['--workers=%d' % (os.cpu_count() - 2)]
+    else:
+      workers_flag = []
+    mac_impeller_unittests_flags = repeat_flags + workers_flag + [
         '--gtest_filter=-*OpenGLES',  # These are covered in the golden tests.
         '--',
         '--enable_vulkan_validation',