tests: fix test_low_fd_limit failing on RLIM_INFINITY platforms

When RLIMIT_NOFILE hard limit is RLIM_INFINITY (macOS default) or
larger than UINT32_MAX, passing limits[1] and limits[1]+1 directly as
--dev-fd-limit-multiplier (a u32 option) causes lightningd to reject
the argument as out-of-range and exit(1), making both nodes fail to
start with "Unable to find Server started with public key" timeout.

Cap to TEST_CEILING=65536 when the hard limit is RLIM_INFINITY or
exceeds the ceiling, keeping the existing soft==hard halving path for
normal bounded limits.

Also add the test to the macOS CI list, so the platform it was broken
on now covers it.

Changelog-None

.github/workflows/macos.yaml

@@ -60,6 +60,7 @@ jobs:
           PYTEST_OPTS: "-vvv --timeout=1800 --durations=10"
           PYTEST_TESTS: |
             tests/test_misc.py::test_ipv4_and_ipv6
+            tests/test_misc.py::test_low_fd_limit
             tests/test_connection.py::test_websocket
             tests/test_connection.py::test_wss_proxy
             tests/test_plugin.py::test_inline_plugin_wait_for_log_no_selfmatch

tests/test_misc.py

@@ -4998,8 +4998,16 @@ def test_set_feerate_offset(node_factory, bitcoind):
 def test_low_fd_limit(node_factory, bitcoind):
     limits = resource.getrlimit(resource.RLIMIT_NOFILE)
 
-    # We assume this, otherwise l2 cannot increase limits!
-    if limits[0] == limits[1]:
+    # dev-fd-limit-multiplier is a u32, so values > UINT32_MAX fail option
+    # parsing. macOS also reports RLIM_INFINITY as the hard limit, making
+    # "ask for more than the hard limit" meaningless.  Cap to a bounded
+    # ceiling so the test works on any platform.
+    TEST_CEILING = 65536
+    if limits[1] == resource.RLIM_INFINITY or limits[1] > TEST_CEILING:
+        limits = (TEST_CEILING // 2, TEST_CEILING)
+        resource.setrlimit(resource.RLIMIT_NOFILE, limits)
+    elif limits[0] == limits[1]:
+        # We assume this, otherwise l2 cannot increase limits!
         limits = (limits[1] // 2, limits[1])
         resource.setrlimit(resource.RLIMIT_NOFILE, limits)