X wcfs: tests: Factor-out waiting for a general condition to become true into waitfor

Currently in wcfs_test.py there is only waiting for a proc
(subprocess.Popen instance) to become ready. However in the next patch
we'll need to wait via polling for another condition.

-> Generalize the pollwait code into waitfor* variants, and make
procwait* use waitfor* internally.

wcfs/wcfs_test.py

@@ -1833,24 +1833,34 @@ def ready(ch):
 
 # procwait waits for a process (subprocess.Popen) to terminate.
 def procwait(ctx, proc):
+    waitfor(ctx, lambda: proc.poll() is not None)
+
+# procwait_, similarly to procwait, waits for a process (subprocess.Popen) to terminate.
+#
+# it returns bool whether process terminated or not - e.g. due to context being canceled.
+def procwait_(ctx, proc):   # -> ok
+    return waitfor_(ctx, lambda: proc.poll() is not None)
+
+# waitfor waits for condf() to become true.
+def waitfor(ctx, condf):
     wg = sync.WorkGroup(ctx)
     def _(ctx):
         while 1:
             if ready(ctx.done()):
                 raise ctx.err()
-            ret = proc.poll()
-            if ret is not None:
+            if condf():
                 return
             tdelay()
     wg.go(_)
     wg.wait()
 
-# procwait_, similarly to procwait, waits for a process (subprocess.Popen) to terminate.
+# waitfor_, similarly to waitfor, waits for condf() to become true.
 #
-# it returns bool whether process terminated or not - e.g. due to context being canceled.
-def procwait_(ctx, proc):   # -> ok
+# it returns bool whether target condition was reached or not - e.g. due to
+# context being canceled.
+def waitfor_(ctx, proc):   # -> ok
     try:
-        procwait(ctx, proc)
+        waitfor(ctx, proc)
     except Exception as e:
         if errors.Is(e, context.canceled) or errors.Is(e, context.deadlineExceeded):
             return False