security/py-keyrings.alt: Add test support to assist committer QA

Add TEST_DEPENDS and (do-)test target to help QA of an upcoming devel/py-fs
update [1].

The tests currently pass: 128 passed, 61 skipped in 21.56 seconds

While I'm here:

• Pet portlint (extra item placed in the USES/USE_x section)

PR: 234491
Approved by: portmgr (framework/infrastructure support/compliance)
MFH: 2019Q1