Fix UT test_BlockUseSwsssdk()

[liuh-80]

Why I did it

Fix this issue: sonic-net/sonic-buildimage#12558
UT failed under python2: test_BlockUseSwsssdk()

  result = subprocess.run(["python", "-c", "import swsssdk;exit()"], capture_output=True, cwd=swsssdk_path)

E AttributeError: 'module' object has no attribute 'run'

How I did it

Update UT to use subprocess.check_output(), which support by both python3 and python2.

How to verify it

Pass all UT.

Which release branch to backport (provide reason below if selected)

Description for the changelog

Fix UT test_BlockUseSwsssdk()

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[liuh-80]

Tested with this PR: sonic-net/sonic-buildimage#12565