bpo-37707: Exclude expensive unit tests from PGO task (GH-15009)

Mark some individual tests to skip when --pgo is used.  The tests
marked increase the PGO task time significantly and likely don't
help improve optimization of the final executable.
diff --git a/Lib/test/libregrtest/main.py b/Lib/test/libregrtest/main.py
index 2b6607d..87278d0 100644
--- a/Lib/test/libregrtest/main.py
+++ b/Lib/test/libregrtest/main.py
@@ -643,6 +643,7 @@
             input("Press any key to continue...")
 
         support.PGO = self.ns.pgo
+        support.PGO_EXTENDED = self.ns.pgo_extended
 
         setup_tests(self.ns)