testsuite: use `skip_' for skipping of tests

testsuite: use `skip_' for skipping of tests

* tests/defs: Use the `skip_' subroutine for test skipping.  Also
give sometimes more detailed messages about the reasons for the
skipping.
* tests/canon7.test: Likewise.
* tests/color.test: Likewise.
* tests/color2.test: Likewise.
* tests/compile2.test: Likewise.
* tests/dejagnu7.test: Likewise.
* tests/depcomp6.test: Likewise.
* tests/depcomp9.test: Likewise.
* tests/depcomp10.test: Likewise.
* tests/distlinks.test: Likewise.
* tests/distlinksbrk.test: Likewise.
* tests/fn99.test: Likewise.
* tests/fn99subdir.test: Likewise.
* tests/forcemiss2.test: Likewise.
* tests/fort5.test: Likewise.
* tests/gettext3.test: Likewise.
* tests/install2.test: Likewise.
* tests/instfail-info.test: Likewise.
* tests/instfail-java.test: Likewise.
* tests/instfail-libtool.test: Likewise.
* tests/instfail.test: Likewise.
* tests/instmany-mans.test: Likewise.
* tests/instmany-python.test: Likewise.
* tests/instmany.test: Likewise.
* tests/instsh3.test: Likewise.
* tests/ltinit.test: Likewise.
* tests/makej2.test: Likewise.
* tests/mdate6.test: Likewise.
* tests/mkinst3.test: Likewise.
* tests/parallel-tests3.test: Likewise.
* tests/parallel-tests-reset-term.test: Likewise.
* tests/parallel-tests-unreadable-log.test: Likewise,
* tests/python-virtualenv.test: Likewise.
* tests/remake-gnulib-remove-header.test: Likewise.
* tests/subobj9.test: Likewise.
* tests/symlink2.test: Likewise.
* tests/tar.test: Likewise.
* tests/tar2.test: Likewise.
* tests/txinfo26.test: Likewise.
* tests/vala2.test: Likewise.
* tests/vala3.test: Likewise.
* tests/vala5.test: Likewise.
* tests/vtexi4.test: Likewise.
* tests/instdir-texi.test: Likewise.
* tests/txinfo21.test: Likewise.

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