tests: beware of Windows file name constraints

Don't expect to be able to build a file named '"\"".y' (6 characters)
on Windows.

Reported by Jannick.
http://lists.gnu.org/archive/html/bug-bison/2018-10/msg00042.html

* tests/synclines.at (syncline escapes): Skip if we failed to create
the file.

tests/synclines.at

@@ -418,6 +418,9 @@ exp: '0'
 ]AT_MAIN_DEFINE[
 ]])
 
+# Maybe we tried to create a file name that the file system does not
+# support.
+AT_SKIP_IF([[test ! -f \"\\\"\".y]])
 AT_FULL_COMPILE([\"\\\"\"])
 AT_BISON_OPTION_POPDEFS