Don't exclude fixed MRI test in test_array.rb file

Author: andrykonchin

test/mri/excludes/TestArray.rb

@@ -46,7 +46,6 @@
 exclude :test_slice_out_of_range, "needs investigation"
 exclude :test_sort_uncomparable, "needs investigation"
 exclude :test_uniq, "needs investigation"
-exclude :test_slice, "needs investigation"
 exclude :test_freeze_inside_sort!, "needs investigation"
 exclude :test_intersect_big_array, "needs investigation"
 exclude :test_intersect?, "needs investigation"