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/test_bz2.py b/Lib/test/test_bz2.py
index f158b90..eb2f72e 100644
--- a/Lib/test/test_bz2.py
+++ b/Lib/test/test_bz2.py
@@ -643,6 +643,7 @@
         data += bz2c.flush()
         self.assertEqual(ext_decompress(data), self.TEXT)
 
+    @support.skip_if_pgo_task
     @bigmemtest(size=_4G + 100, memuse=2)
     def testCompress4G(self, size):
         # "Test BZ2Compressor.compress()/flush() with >4GiB input"
@@ -701,6 +702,7 @@
         self.assertRaises(EOFError, bz2d.decompress, b"anything")
         self.assertRaises(EOFError, bz2d.decompress, b"")
 
+    @support.skip_if_pgo_task
     @bigmemtest(size=_4G + 100, memuse=3.3)
     def testDecompress4G(self, size):
         # "Test BZ2Decompressor.decompress() with >4GiB input"