Update P3GExtensionsTest.class.st to fix ordered/unordered issue

[svenvc]

P3GExtensionsTest>>#testDictionaryWritePython3On and P3GExtensionsTest>>#testSetWritePython3On assume an order that does not exist (Set and Dict are unordered), which makes them fail (for me at least).

Since this test is only about writing and checking the expected output, the simplest solution is to use only one element.