Fixed so that the test of fork may operate correctly.

The test of fork seems not to operate correctly.

	$ perl -Ilib t/Builder/fork_with_new_stdout.t 
	1..2
	You tried to plan twice at t/Builder/fork_with_new_stdout.t line 38.
	Use of uninitialized value $pipe in pattern match (m//) at t/Builder/fork_with_new_stdout.t line 27.
	ok 1
	Use of uninitialized value $pipe in pattern match (m//) at t/Builder/fork_with_new_stdout.t line 28.
	ok 2

In addition, in this pull-request, although it changed so that 'done_testing' might be used, I regard as another problem that 'no_plan' does not print the plan number to last line.