Fix: shut up crystal exhaustive case warnings

In one case the compiler is wrong: the case is exhaustive. The other
two cases are noise.

src/assertions.cr

@@ -309,11 +309,13 @@ module Minitest
       case stdout
       when String then assert_equal stdout, output
       when Regex then assert_match stdout, output
+      when Nil # skip
       end
 
       case stderr
       when String then assert_equal stderr, error
       when Regex then assert_match stderr, error
+      when Nil # skip
       end
     end
 

src/reporter.cr

@@ -161,6 +161,8 @@ module Minitest
           when Skip
             puts "  #{i + 1}) Skipped:".colorize(:yellow)
             puts "#{loc} [#{exception.__minitest_location}]:\n#{exception.message}"
+          else
+            # shut up, crystal (you're wrong)
           end
           puts
         end