tests: add optional cursor to log_contains + fix truthiness issues in callers (#6960)

Extracted from https://github.com/neondatabase/neon/pull/6953

Part of https://github.com/neondatabase/neon/issues/5899

Core Change
-----------

In #6953, we need the ability to scan the log _after_ a specific line
and ignore anything before that line.

This PR changes `log_contains` to returns a tuple of `(matching line,
cursor)`.
Hand that cursor to a subsequent `log_contains` call to search the log
for the next occurrence of the pattern.

Other Changes
-------------

- Inspect all the callsites of `log_contains` to handle the new tuple
return type.
- Above inspection unveiled many callers aren't using `assert
log_contains(...) is not None` but some weaker version of the code that
breaks if `log_contains` ever returns a not-None but falsy value. Fix
that.
- Above changes unveiled that `test_remote_storage_upload_queue_retries`
was using `wait_until` incorrectly; after fixing the usage, I had to
raise the `wait_until` timeout. So, maybe this will fix its flakiness.

test_runner/fixtures/utils.py

@@ -369,7 +369,12 @@ def start_in_background(
         return spawned_process
 
 
-def wait_until(number_of_iterations: int, interval: float, func: Fn):
+WaitUntilRet = TypeVar("WaitUntilRet")
+
+
+def wait_until(
+    number_of_iterations: int, interval: float, func: Callable[[], WaitUntilRet]
+) -> WaitUntilRet:
     """
     Wait until 'func' returns successfully, without exception. Returns the
     last return value from the function.
@@ -387,6 +392,18 @@ def wait_until(number_of_iterations: int, interval: float, func: Fn):
     raise Exception("timed out while waiting for %s" % func) from last_exception
 
 
+def assert_eq(a, b) -> None:
+    assert a == b
+
+
+def assert_gt(a, b) -> None:
+    assert a > b
+
+
+def assert_ge(a, b) -> None:
+    assert a >= b
+
+
 def run_pg_bench_small(pg_bin: "PgBin", connstr: str):
     """
     Fast way to populate data.