In run_tests, use the regular path to the Vulkan library if lib.unstripped does not exist (#50629)

lib.unstripped will not be created in a "gn --no-stripped" build

See https://github.com/flutter/engine/pull/50443
diff --git a/testing/run_tests.py b/testing/run_tests.py
index 904790d..c818487 100755
--- a/testing/run_tests.py
+++ b/testing/run_tests.py
@@ -238,11 +238,13 @@
     env['LD_LIBRARY_PATH'] = build_dir
     env['VK_DRIVER_FILES'] = os.path.join(build_dir, 'vk_swiftshader_icd.json')
     if os.path.exists(unstripped_exe):
+      unstripped_vulkan = os.path.join(build_dir, 'lib.unstripped', 'libvulkan.so.1')
+      if os.path.exists(unstripped_vulkan):
+        vulkan_path = unstripped_vulkan
+      else:
+        vulkan_path = os.path.join(build_dir, 'libvulkan.so.1')
       try:
-        os.symlink(
-            os.path.join(build_dir, 'lib.unstripped', 'libvulkan.so.1'),
-            os.path.join(build_dir, 'exe.unstripped', 'libvulkan.so.1')
-        )
+        os.symlink(vulkan_path, os.path.join(build_dir, 'exe.unstripped', 'libvulkan.so.1'))
       except OSError as err:
         if err.errno == errno.EEXIST:
           pass