yacc: extend and improve tests * tests/yacc-basic.test: Also check that the intermediate C file is mentioned in the generated Makefile.in, and that it is created by the first make invocation. * tests/yacc3.test: Test removed, superseded by ... * tests/yacc-d-basic.test: ... this new test. * tests/yacc2.test: Add reference to that new test in the heading comments. * tests/yacc-d-vpath.test: New test. * tests/yaccvpath.test: Updated heading comments. Do not require gcc anymore, as any working C compiler should be enough. Remove redundant comments. * tests/yacc-nodist.test: New test. * tests/yacc-dist-nobuild.test: New test. * tests/Makefile.am (TESTS): Update.