assert() -> doAssert()

Also fixed a test and added the other to the Nimble task.

tests/test_input_stream.nim

@@ -3,7 +3,7 @@ import
   ../faststreams
 
 suite "input stream":
-  test "string output":
+  test "string input":
     var input = repeat("1234 5678 90AB CDEF\n", 1000)
     var stream = memoryStream(input)
 

tests/test_output_stream.nim

@@ -17,5 +17,6 @@ suite "output stream":
       altOutput.add '\n'
 
     let streamOutput = s.getOutput
-    check streamOutput == altOutput
+    # without the cast, in nim-0.19.0: "Error: type mismatch: got <seq[byte], string>" (for the `==` proc)
+    check cast[string](streamOutput) == altOutput