Have targets set pw_unit_test_create_run_targets

- Move pw_unit_test_create_run_targets to pw_vars_default.gni so that
  targets can set it.
- Keep the default false, but set it to true for the host targets.

Change-Id: Ia1af584642d0dd4d16b801b2ee4a8bb1a78c286e
diff --git a/pw_vars_default.gni b/pw_vars_default.gni
index f0fd89f..9f2019e 100644
--- a/pw_vars_default.gni
+++ b/pw_vars_default.gni
@@ -44,3 +44,13 @@
 
 # Implementation of a main function for "pw_test" unit test binaries.
 pw_unit_test_main = "$dir_pw_unit_test:main"
+
+# Whether GN unit test runner targets should be created.
+#
+# If set to true, the pw_test() template creates an action that invokes the test
+# runner script on each test executable. If false, the pw_test() template only
+# creates a test executable target.
+#
+# This should be enabled for targets which support parallelized running
+# of unit tests, such as desktops with multiple cores.
+pw_unit_test_create_run_targets = false