Tweak `do_ctlc()`'s `delay` default

To be a null default and set to `0.1` when not passed by the caller so
as to avoid having to pass `0.1` if you wanted the
param-defined-default.

Also,
- in the `spawn()` fixtures's `unset_colors()` closure, add in a masked
  `os.environ['NO_COLOR'] = '1'` since i found it while trying to debug
  debugger tests.
- always return the `child.before` content from `assert_before()`
  helper; again it comes in handy when debugging console matching tests.

tests/devx/conftest.py

@@ -68,7 +68,10 @@ def spawn(
 
         '''
         import os
+        # disable colored tbs
         os.environ['PYTHON_COLORS'] = '0'
+        # disable all ANSI color output
+        # os.environ['NO_COLOR'] = '1'
 
     spawned: PexpectSpawner|None = None
 
@@ -251,12 +254,13 @@ def assert_before(
         err_on_false=True,
         **kwargs
     )
+    return str(child.before.decode())
 
 
 def do_ctlc(
     child,
     count: int = 3,
-    delay: float = 0.1,
+    delay: float|None = None,
     patt: str|None = None,
 
     # expect repl UX to reprint the prompt after every
@@ -268,6 +272,7 @@ def do_ctlc(
 ) -> str|None:
 
     before: str|None = None
+    delay = delay or 0.1
 
     # make sure ctl-c sends don't do anything but repeat output
     for _ in range(count):
@@ -278,7 +283,10 @@ def do_ctlc(
         # if you run this test manually it works just fine..
         if expect_prompt:
             time.sleep(delay)
-            child.expect(PROMPT)
+            child.expect(
+                PROMPT,
+                # timeout=1,  # TODO? if needed
+            )
             before = str(child.before.decode())
             time.sleep(delay)