Conditionalize use statements for fromxml, fromatomfeed, fromdb, and todb

The dependencies of these operations are no longer default prereqs since
5d48695 and 422b554.
Extend OperationHelper to skip tests when optional operation deps are missing

This is a cleaner spot to do optional dep checking in tests than copy
and pasting specific operation deps and the code to check for them into
each test script.
Correctly report test failures at the callsite in the testfile

Otherwise failure diagnostics come from within the test libraries and
make finding the failing test in the testfile itself harder than it
should be.
Skip tests for operations/components with optional deps

Since the move to a pure-Perl set of core required deps (with
specific-operational deps moved into features), fromxml, fromatomfeed,
fromdb, and todb are no longer guaranteed dep satisfaction.  (But if
they try sometimes, well they might just find, they get what they need.)
Text::CSV's pure-Perl implementation doesn't correctly handle embedded newlines + allow_loose_quotes

The XS implementation works fine, so only skip the failing tests if we
we're not using Text::CSV_XS.  I reported this bug as [rt.cpan.org #99589]:

    https://rt.cpan.org/Ticket/Display.html?id=99589

Most of the diff for this change is whitespace, so best viewed with
whitespace ignored.
Run the test suite under minimal deps if we're an author

Author tests are run under `milla test`.
Merge pull request #50 from benbernard/optional-test-deps

Optional test deps