Package rpminstall-tests-checkinstall: Information

- To catch more errors (evidenced by file conflicts during rpm -U),
  now the package template includes a file with a unique value.
  (This runs more slowly.) (Suggested by Vladimir Seleznev.)

- (.spec) Factored out /usr/sbin/sh-safely (checkinstall-helper-sh-safely pkg)
  from %pre of the checkinstall subpkg.
- In the tests, used the common /bin/tmpdir.sh (from tmpdir.sh pkg).

- New tests for disttag comparison with the obsolete (.) and current format (+)
  (Correct comparison would rely on a fix or a new feature in rpm:
  %_priority_distbranch.)
- New tests for the %_priority_distbranch feature in normal situation.
- The reasons of the currently XFAILing tests (with rpm-4.13.0.1-alt6):
  1. upgrade w.r.t. buildtime is not a strict order;
  2. upgrade w.r.t. disttag:
    2a. not a strict order;
    2b. %_priority_distbranch not honored;
    2c. unrecognized disttag format is not "older"
        than a disttag with a recognized branch prefix;
  3. upgrade w.r.t. disttag fails in non-standard configuration
     without honor_buildtime;
  4. obsoleting an exact disttag (which is an unrealistic situation)
     doesn't work;
  5. mishandled elusive dep with a release but no epoch.