Skip tests that fail specifically on Cygwin.

Right now this is the only such test. There are other tests that generate code that uses long double math functions, but not in a way that appears to break the test.

runtests.py

@@ -1993,10 +1993,17 @@ def runtests(options, cmd_args, coverage=None):
         exclude_selectors.append(ShardExcludeSelector(options.shard_num, options.shard_count))
 
     if not test_bugs:
+        bug_files = [
+            ('bugs.txt', True),
+            ('pypy_bugs.txt', IS_PYPY),
+            ('windows_bugs.txt', sys.platform == 'win32'),
+            ('cygwin_bugs.txt', sys.platform == 'cygwin')
+        ]
+
         exclude_selectors += [
-            FileListExcluder(os.path.join(ROOTDIR, bugs_file_name), verbose=verbose_excludes)
-            for bugs_file_name in ['bugs.txt'] + (['pypy_bugs.txt'] if IS_PYPY else []) +
-            (['windows_bugs.txt'] if sys.platform == 'win32' else [])
+            FileListExcluder(os.path.join(ROOTDIR, bugs_file_name),
+                             verbose=verbose_excludes)
+            for bugs_file_name, condition in bug_files if condition
         ]
 
     if sys.platform in ['win32', 'cygwin'] and sys.version_info < (2,6):

tests/cygwin_bugs.txt

+int_float_builtins_as_casts_T400_long_double

tests/run/int_float_builtins_as_casts_T400.pyx

@@ -187,24 +187,6 @@ def double_to_float_int(double x):
     return r
 
 
-@cython.test_fail_if_path_exists("//SingleAssignmentNode//TypecastNode")
-@cython.test_assert_path_exists(
-    "//PythonCapiCallNode",
-    "//PythonCapiCallNode/PythonCapiFunctionNode/@cname = 'truncl'",
-)
-def long_double_to_float_int(long double x):
-    """
-    >>> long_double_to_float_int(4.1)
-    4.0
-    >>> long_double_to_float_int(-4.1)
-    -4.0
-    >>> long_double_to_float_int(4)
-    4.0
-    """
-    cdef float r = int(x)
-    return r
-
-
 @cython.test_fail_if_path_exists("//SimpleCallNode")
 @cython.test_assert_path_exists("//PythonCapiCallNode")
 def object_float(x):

tests/run/int_float_builtins_as_casts_T400_long_double.pyx

+# ticket: 400
+
+cimport cython
+
+
+@cython.test_fail_if_path_exists("//SingleAssignmentNode//TypecastNode")
+@cython.test_assert_path_exists(
+    "//PythonCapiCallNode",
+    "//PythonCapiCallNode/PythonCapiFunctionNode/@cname = 'truncl'",
+)
+def long_double_to_float_int(long double x):
+    """
+    >>> long_double_to_float_int(4.1)
+    4.0
+    >>> long_double_to_float_int(-4.1)
+    -4.0
+    >>> long_double_to_float_int(4)
+    4.0
+    """
+    cdef float r = int(x)
+    return r