test/refactor: use test deque to avoid quadratic iteration

Extracted from https://github.com/bitcoin/bitcoin/pull/33141#discussion_r2323012972.
In Python, list `pop(0)` is linear, so consuming all items is quadratic.
Switched to `collections.deque` with `popleft()` to express FIFO intent and avoid the O(n^2) path.
Behavior is unchanged; for a few hundred items the perf impact is likely negligible.

Ref: https://docs.python.org/3/tutorial/datastructures.html#using-lists-as-queues
> While appends and pops from the end of list are fast, doing inserts or pops
> from the beginning of a list is slow (because all of the other elements have
> to be shifted by one).

Co-authored-by: maflcko <6399679+maflcko@users.noreply.github.com>

test/functional/test_runner.py

@@ -452,8 +452,8 @@ def main():
         print("Re-compile with the -DBUILD_DAEMON=ON build option")
         sys.exit(1)
 
-    # Build list of tests
-    test_list = []
+    # Build tests
+    test_list = deque()
     if tests:
         # Individual tests have been specified. Run specified tests that exist
         # in the ALL_SCRIPTS list. Accept names with or without a .py extension.
@@ -472,7 +472,7 @@ def main():
             script = script + ".py" if ".py" not in script else script
             matching_scripts = [s for s in ALL_SCRIPTS if s.startswith(script)]
             if matching_scripts:
-                test_list.extend(matching_scripts)
+                test_list += matching_scripts
             else:
                 print("{}WARNING!{} Test '{}' not found in full test list.".format(BOLD[1], BOLD[0], test))
     elif args.extended:
@@ -507,7 +507,7 @@ def main():
                 remove_tests([test for test in test_list if test.split('.py')[0] == exclude_test.split('.py')[0]])
 
     if args.filter:
-        test_list = list(filter(re.compile(args.filter).search, test_list))
+        test_list = deque(filter(re.compile(args.filter).search, test_list))
 
     if not test_list:
         print("No valid test scripts specified. Check that your test is in one "
@@ -724,7 +724,7 @@ class TestHandler:
     def get_next(self):
         while len(self.jobs) < self.num_jobs and self.test_list:
             # Add tests
-            test = self.test_list.pop(0)
+            test = self.test_list.popleft()
             portseed = len(self.test_list)
             portseed_arg = ["--portseed={}".format(portseed)]
             log_stdout = tempfile.SpooledTemporaryFile(max_size=2**16)
@@ -752,8 +752,7 @@ class TestHandler:
             ]
             fut = self.executor.submit(proc_wait, task)
             self.jobs[fut] = test
-        if not self.jobs:
-            raise IndexError('pop from empty list')
+        assert self.jobs  # Must not be empty here
 
         # Print remaining running jobs when all jobs have been started.
         if not self.test_list: