Extend, fix and improve tests on Lex and Yacc support.

Extend, fix and improve tests on Lex and Yacc support.

* tests/lexcpp.test: New test script, on support for Lex + C++.
* tests/lexvpath.test: New test script, test build and rebuild
rules for lexers in VPATH setup.
* tests/yacc-basic.test: New test script, run simple "semantic"
checks on basic Yacc support (similarly to what lex3.test does
for Lex support).
* tests/lex.test: Don't create useless dummy source file joe.l.
Remove extra blank lines.
* tests/lex4.test: Add trailing `:' command.  Do not create dummy
useless lex source file.
* tests/lex2.test: Likewise.  Call automake with the `-a' option,
so that it doesn't fail for the absence of `ylwrap' script.  Make
grepping of automake stderr stricter.
* tests/yacc7.test: Add trailing `:' command.  Enable `errexit'
shell flag earlier (just after having sourced ./defs).
* tests/yacc4.test: Likewise.  Also ...
(configure.in): Use pre-populated skeleton set up by ./defs,
instead of writing one from scratch.
Other minor cosmetic changes.
* tests/yacc5.test: Likewise.
* tests/yaccvpath.test: Likewise. Also ...
($distdir): New variable.
Use it throughout.
* tests/lex5.test: Likewise.
* tests/lex3.test: Likewise.  Check the distdir, rather than
grepping the distribution tarball.  Extend the test on the
created binary, and be sure to avoid hangs.  Add some comments.
* tests/yacc.test: Use stricter grepping.  Add trailing `:'.
* tests/yacc6.test: Likewise.
* tests/yacc3.test: Likewise.  Do not create the unused file
`Makefile.sed'.  Remove useless rules from Makefile.am.  Other
minor cosmetic changes.
* tests/yacc2.test: Make grepping of generated `Makefile.in' and
of automake error messages stricter.  Do not redirect output of
grep to /dev/null.  Move call to aclocal earlier.  Reduce the
number of empty blank lines.  Fix a typo in comments.
* tests/yacc8.test: Fixed bugs that reduced the completeness of
the tests.  Added trailing `:' command.
(configure.in): Use pre-populated skeleton set up by ./defs,
instead of writing one from scratch.
* tests/yaccpp.test: Test also extensions `.y++', `.ypp', and
`.yxx', rather than only `.yy'.
* tests/Makefile.am (TESTS): Update.