tests: make neon_fixtures a bit thinner by splitting out some pageserver related helpers (#3977)

neon_fixture is quite big and messy, lets clean it up a bit.

test_runner/fixtures/utils.py

@@ -278,3 +278,19 @@ def wait_until(number_of_iterations: int, interval: float, func: Fn):
             continue
         return res
     raise Exception("timed out while waiting for %s" % func) from last_exception
+
+
+def wait_while(number_of_iterations: int, interval: float, func):
+    """
+    Wait until 'func' returns false, or throws an exception.
+    """
+    for i in range(number_of_iterations):
+        try:
+            if not func():
+                return
+            log.info("waiting for %s iteration %s failed", func, i + 1)
+            time.sleep(interval)
+            continue
+        except Exception:
+            return
+    raise Exception("timed out while waiting for %s" % func)