Add metavar to match var name in help text + Change wording for better readability

Summary:

The help text given by test/functional/test_runner.py -h refers to the value n, which is defined as COMBINEDLOGSLEN in the list of commands.

To make the help text consistent, this PR changes the display name COMBINEDLOGSLEN to n by setting the argparse metavar attribute. (metavar only changes the displayed name)

This is a backport of Core PR14619

Test Plan:

$ test/functional/test_runner.py -h | grep -A 3 combinedlogslen
  --combinedlogslen n, -c n
                        On failure, print a log (of length n lines) to the
                        console, combined from the test framework and all test
                        nodes. (default: 0)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8347