terminalwriter: don't flush implicitly; add explicit flushes

Flushing on every write is somewhat expensive.

Rely on line buffering instead (if line buffering for stdout is
disabled, there must be some reason...), and add explicit flushes when
not outputting lines.

This is how regular `print()` e.g. work so should be familiar.

src/_pytest/setuponly.py

@@ -68,6 +68,8 @@ def _show_fixture_action(fixturedef, msg):
     if hasattr(fixturedef, "cached_param"):
         tw.write("[{}]".format(fixturedef.cached_param))
 
+    tw.flush()
+
     if capman:
         capman.resume_global_capture()