Allow musl libc's -1 error message during testing

Closes #920

changelog.rst

@@ -11,6 +11,8 @@ Changelog
 - [documentation] HTML documentation builds are now tested for errors.
 - [fsevents2] The fsevents2 observer is now deprecated.
 - [watchmedo] Handle shutdown events from ``SIGHUP`` (`#912 <https://github.com/gorakhargosh/watchdog/pull/912>`__)
+- [tests] The error message returned by musl libc for error code ``-1`` is now allowed.
+  (`#920 <https://github.com/gorakhargosh/watchdog/issues/920>`_)
 - Thanks to our beloved contributors: @kurtmckee @babymastodon
 
 2.1.9

tests/test_inotify_c.py

@@ -135,18 +135,20 @@ def inotify_rm_watch(fd, wd):
     assert inotify_fd.wds == [2, 3]  # Only 1 is removed explicitly
 
 
-@pytest.mark.parametrize("error, pattern", [
-    (errno.ENOSPC, "inotify watch limit reached"),
-    (errno.EMFILE, "inotify instance limit reached"),
-    (errno.ENOENT, "No such file or directory"),
-    (-1, "Unknown error -1"),
+@pytest.mark.parametrize("error, patterns", [
+    (errno.ENOSPC, ("inotify watch limit reached",)),
+    (errno.EMFILE, ("inotify instance limit reached",)),
+    (errno.ENOENT, ("No such file or directory",)),
+    # Error messages for -1 are undocumented
+    # and vary between libc implementations.
+    (-1, ("Unknown error -1", "No error information")),
 ])
-def test_raise_error(error, pattern):
+def test_raise_error(error, patterns):
     with patch.object(ctypes, "get_errno", new=lambda: error):
         with pytest.raises(OSError) as exc:
             Inotify._raise_error()
     assert exc.value.errno == error
-    assert pattern in str(exc.value)
+    assert any(pattern in str(exc.value) for pattern in patterns)
 
 
 def test_non_ascii_path():