tests: fix spurious failure with FreeBSD make and Yacc in VPATH * tests/subpkg.test: Some cosmetic adjustments. Move the tests checking that $(YLWRAP) is defined and installed properly when ylwrap is in a default auxdir found in a parent package ... * tests/subpkg-yacc.test: ... into this new test, which carefully avoids to trigger the known bug#7884 (combo FreeBSD make plus Yacc plus VPATH build). * tests/Makefile.am (TESTS): Update.