tests/py: Add a test sanitizer and fix its findings

tests/py: Add a test sanitizer and fix its findings

This is just basic housekeeping:

- Remove duplicate tests in any of the *.t files
- Remove explicit output if equal to command itself in *.t files
- Remove duplicate payload records in any of the *.t.payload* files
- Remove stale payload records (for which no commands exist in the
  respective *.t file
- Remove duplicate/stale entries in any of the *.t.json files

In some cases, tests were added instead of removing a stale payload
record if it fit nicely into the sequence of tests.

Signed-off-by: Phil Sutter <phil@nwl.cc>