Add retry to some tests (#689)

Author: slozier

Src/IronPythonTest/Cases/IronPythonCasesManifest.ini

@@ -158,6 +158,9 @@ Arguments=-X:CompilationThreshold 0 "$(TEST_FILE)" # ensure CompilationThreshold
 [IronPython.test_setcomp]
 IsolationLevel=PROCESS # https://github.com/IronLanguages/ironpython3/issues/489
 
+[IronPython.test_shutil]
+RetryCount=2
+
 [IronPython.test_slice]
 Ignore=true
 
@@ -249,6 +252,9 @@ Ignore=true
 [IronPython.modules.io_related.test_marshal]
 Ignore=true
 
+[IronPython.modules.misc.test__weakref]
+RetryCount=2
+
 [IronPython.modules.misc.test_future]
 Ignore=true