test-lib-functions: introduce the 'test_set_port' helper function

Several test scripts run daemons like 'git-daemon' or Apache, and
communicate with them through TCP sockets.  To have unique ports where
these daemons are accessible, the ports are usually the number of the
corresponding test scripts, unless the user overrides them via
environment variables, and thus all those tests and test libs contain
more or less the same bit of one-liner boilerplate code to find out
the port.  The last patch in this series will make this a bit more
complicated.

Factor out finding the port for a daemon into the common helper
function 'test_set_port' to avoid repeating ourselves.

Take special care of test scripts with "low" numbers:

  - Test numbers below 1024 would result in a port that's only usable
    as root, so set their port to '10000 + test-nr' to make sure it
    doesn't interfere with other tests in the test suite.  This makes
    the hardcoded port number in 't0410-partial-clone.sh' unnecessary,
    remove it.

  - The shell's arithmetic evaluation interprets numbers with leading
    zeros as octal values, which means that test number below 1000 and
    containing the digits 8 or 9 will trigger an error.  Remove all
    leading zeros from the test numbers to prevent this.

Note that the 'git p4' tests are unlike the other tests involving
daemons in that:

  - 'lib-git-p4.sh' doesn't use the test's number for unique port as
    is, but does a bit of additional arithmetic on top [1].

  - The port is not overridable via an environment variable.

With this patch even 'git p4' tests will use the test's number as
default port, and it will be overridable via the P4DPORT environment
variable.

[1] Commit fc00233071 (git-p4 tests: refactor and cleanup, 2011-08-22)
    introduced that "unusual" unique port computation without
    explaining why it was necessary (as opposed to simply using the
    test number as is).  It seems to be just unnecessary complication,
    and in any case that commit came way before the "test nr as unique
    port" got "standardized" for other daemons in commits c44132fcf3
    (tests: auto-set git-daemon port, 2014-02-10), 3bb486e439 (tests:
    auto-set LIB_HTTPD_PORT from test name, 2014-02-10), and
    bf9d7df950 (t/lib-git-svn.sh: improve svnserve tests with parallel
    make test, 2017-12-01).

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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