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_lzma.py b/Lib/test/test_lzma.py
index 3dc2c1e..117de0a 100644
--- a/Lib/test/test_lzma.py
+++ b/Lib/test/test_lzma.py
@@ -333,6 +333,7 @@
 
     # Test with inputs larger than 4GiB.
 
+    @support.skip_if_pgo_task
     @bigmemtest(size=_4G + 100, memuse=2)
     def test_compressor_bigmem(self, size):
         lzc = LZMACompressor()
@@ -344,6 +345,7 @@
         finally:
             ddata = None
 
+    @support.skip_if_pgo_task
     @bigmemtest(size=_4G + 100, memuse=3)
     def test_decompressor_bigmem(self, size):
         lzd = LZMADecompressor()