fix: correctly print counter-example for parallel execution

[thalesmg]

When using parallel execution, the counter-example is a singleton list with a single 2-tuple containing the sequential prefix and the parallel executions. But the code was expecting it to be an enumerable.

  1) property broken ticket issuer (PropCheck.Test.BrokenTicketIssuerTest)
     test/broken_ticket_issuer_test.exs:8
     ** (Protocol.UndefinedError) protocol Enumerable not implemented for {[], [[{:set, {:var, 2}, {:call, PropCheck.Test.BrokenTicketIssuerTest, :inc, []}}, {:set, {:var, 3}, {:call, PropCheck.Test.BrokenTicketIssuerTest, :inc, []}}, {:set, {:var, 5}, {:call, PropCheck.Test.BrokenTicketIssuerTest, :inc, []}}], [{:set, {:var, 7}, {:call, PropCheck.Test.BrokenTicketIssuerTest, :inc, []}}, {:set, {:var, 8}, {:call, PropCheck.Test.BrokenTicketIssuerTest, :inc, []}}]]} of type Tuple. This protocol is implemented for the following type(s): Date.Range, File.Stream, Function, GenEvent.Stream, HashDict, HashSet, IO.Stream, List, Map, MapSet, Range, Stream
     code: property "broken ticket issuer", [:verbose] do
     stacktrace:
       (elixir 1.13.4) lib/enum.ex:1: Enumerable.impl_for!/1
       (elixir 1.13.4) lib/enum.ex:143: Enumerable.reduce/3
       (elixir 1.13.4) lib/enum.ex:4144: Enum.reduce/3
       (propcheck 1.4.2-dev) lib/properties.ex:300: PropCheck.Properties.counter_example_inspect/1
       (propcheck 1.4.2-dev) lib/properties.ex:249: PropCheck.Properties.handle_check_results/2
       test/broken_ticket_issuer_test.exs:8: (test)

Also, while running a test several times to confirm the fix, another bug was found for the case when the parallel execution counter-example has an empty sequential prefix. In that case, the counter-example was not being classified as a parallel one, and printed incorrectly.

  1) property broken ticket issuer (PropCheck.Test.BrokenTicketIssuerTest)
     test/broken_ticket_issuer_test.exs:8
     Property Elixir.PropCheck.Test.BrokenTicketIssuerTest.property broken ticket issuer() failed. Counter-Example is:
     [
       {[],
        [
          [
            {:set, {:var, 2},
             {:call, PropCheck.Test.BrokenTicketIssuerTest, :inc, []}},
            {:set, {:var, 3},
             {:call, PropCheck.Test.BrokenTicketIssuerTest, :inc, []}},
            {:set, {:var, 4},
             {:call, PropCheck.Test.BrokenTicketIssuerTest, :inc, []}},
            {:set, {:var, 5},
             {:call, PropCheck.Test.BrokenTicketIssuerTest, :inc, []}}
          ],
          [
            {:set, {:var, 8},
             {:call, PropCheck.Test.BrokenTicketIssuerTest, :inc, []}}
          ]
        ]}
     ]