Prevent TextIOWrapper from buffering echo stream

TextIOWrapper does buffered reads, so the first call to readline (or
read) will read a large chunk from the underlying stream. When that
stream is the EchoingStdin stream, it causes multiple lines of input
to be echoed prematurely.

The solution in this commit is not ideal because it relies on the
undocumented internal _CHUNK_SIZE variable. I couldn't find any other
way to disable read buffering without reimplementing TextIOWrapper.

src/click/testing.py

@@ -210,6 +210,11 @@ def isolation(self, input=None, env=None, color=False):
         sys.stdin = input = _NamedTextIOWrapper(
             input, encoding=self.charset, name="<stdin>", mode="r"
         )
+        if self.echo_stdin:
+            # Force unbuffered reads, otherwise the underlying EchoingStdin
+            # stream will echo a big chunk of input on the first read.
+            input._CHUNK_SIZE = 1
+
         sys.stdout = _NamedTextIOWrapper(
             bytes_output, encoding=self.charset, name="<stdout>", mode="w"
         )