tests: ignore the python libmount tests when they're run under UBSan

tests: ignore the python libmount tests when they're run under UBSan

ts_init_py meddles with LD_LIBRARY_PATH causing those tests to fail with
```
+Traceback (most recent call last):
+  File "/home/travis/build/karelzak/util-linux/libmount/python/test_mount_tab.py", line 8, in <module>
+    import pylibmount as mnt
+ImportError: /home/travis/build/karelzak/util-linux/.libs/libblkid.so.1: undefined symbol: __ubsan_handle_negate_overflow
}}}-diff
```