Modernize, improve and extend tests for PR (`pr[0-9]*.test').

Modernize, improve and extend tests for PR (`pr[0-9]*.test').

* tests/pr2.test: Add trailing `:' command.
* tests/pr229.test: Likewise.
* tests/pr401.test: Likewise.
* tests/pr401b.test: Likewise.
* tests/pr401c.test: Likewise.
* tests/pr300-prog.test: Likewise, plus cosmetic changes in
spacing.
* tests/pr300-lib.test: Likewise.
* tests/pr300-ltlib.test: Likewise, and ensure verbose printing
of captured make stdout.
* tests/pr211.test: Add trailing `:' command.  Also, use the
`configure.in' stub created by ./defs, rather than writing it
from scratch.
* tests/pr204.test: Likewise, plus cosmetic spacing changes.
* tests/pr287.test: Likewise, and move setting of `errexit' shell
flag earlier in the script (just after inclusion of ./defs).
* tests/pr220.test: Make grepping of Automake's stderr stricter.
Also, add trailing `:' command, and cosmetic changes in spacing.
* tests/pr224.test: Move setting of `errexit' shell flag earlier
in the script (just after inclusion of ./defs).  Do not export
`CC=gcc' to configure explicitly (it's already exported globally
in ./defs, since we have "gcc" in $required).  Use the stub for
`configure.in' created by ./defs, rather than writing it from
scratch.  Do not create dummy files required by "gnu" mode (e.g.
README, NEWS), since we run automake in foreign mode anyway.
* tests/pr72.test: Enable `errexit' shell flags, and related
changes.  Extend existing checks a bit.
* tests/pr9.test: Likewise.  Also, avoid obolescent constructs in
the generated `configure.in', and extend existing checks over the
generated tarball a bit.
* tests/pr87.test: Enable `errexit' shell flags, and related
changes.  Add a trailing `:' command.  Also, do not create dummy
files required by "gnu" mode (e.g. README, NEWS), since we run
automake in foreign mode anyway.
* tests/pr243.test: Avoid obolescent constructs in the generated
`configure.in'.  Enable the `errexit' shell flag, and related
changes.  Cosmetic changes to spacing, add trailing `:' command,
and add a "FIXME" comment.
* tests/pr266.test: Likewise, and add explicit command line switch
`--enable-dependecy-tracking' to the ./configure call.
* tests/pr279.test: Avoid obolescent constructs in the generated
`configure.in'; also, use the `configure.in' stub created by
./defs, rather than writing it from scratch.  Enable `errexit'
shell flag, and related changes.  Add trailing `:' command.
* tests/pr279-2.test: Likewise, and make grepping of Makefile.in
stricter.
* tests/pr307.test: Move setting of `errexit' shell flag earlier
in the script (just after inclusion of ./defs).  Escape literal
dots in grep regular expressions.  Also, add a trailing `:'
command, and cosmetic changes to spacing.