lex/yacc tests: remove redundant $distdir definition * tests/lex3.test ($distdir): Remove definition, that's already done in `tests/defs'. * tests/lexvpath.test: Likewise. * tests/yacc-basic.test: Likewise. * tests/yacc-cxx.test: Likewise. * tests/yacc-d-basic.test: Likewise. * tests/yacc-d-cxx.test: Likewise. * tests/yacc-d-vpath.test: Likewise. * tests/yacc-dist-nobuild-subdir.test: Likewise. * tests/yacc-dist-nobuild.test: Likewise. * tests/yacc-mix-c-cxx.test: Likewise. * tests/yaccvpath.test: Likewise.