tests: python tests should not remove user's LD_LIBRARY_PATH

tests: python tests should not remove user's LD_LIBRARY_PATH

LD_LIBRARY_PATH must be preserved like it's done in libtool's
temporary wrapper scripts.

This problem was noticed when building with Intel's icc (15.0.0):
$ make check TS_OPTS="libmount"
$ tail tests/diff/libmount/tabfiles-py-parse-mtab
[...]
+Traceback (most recent call last):
+  File "/tmp/util-linux/libmount/python/test_mount_tab.py", line 8, in <module>
+    import pylibmount as mnt
+ImportError: libimf.so: cannot open shared object file: No such file or directory

... although libimf.so was in LD_LIBRARY_PATH.

NOTE 1:
  When running root checks "sudo -E" is not enough to preserve
  LD_LIBRARY_PATH because glibc already removes is. This would work:
  $ sudo -E env LD_LIBRARY_PATH=$LD_LIBRARY_PATH make check TS_OPTS="--parallel=1"

NOTE 2:
  We also preserve PYTHONPATH which could bring us similar problems if
  the user has a customized python installation.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>