tests: avoid creating useless files in some test directories * tests/aclocal.test: Define `$am_create_testdir' to "empty" before sourcing ./defs, to avoid bringing in unused auxiliary files. Related adjustments. * tests/aclocal3.test: Likewise. * tests/aclocal8.test: Likewise. * tests/aclocal9.test: Likewise. * tests/acloca10.test: Likewise. * tests/acloca11.test: Likewise. * tests/acloca12.test: Likewise. * tests/acloca15.test: Likewise. * tests/acloca16.test: Likewise. * tests/acloca17.test: Likewise. * tests/acloca18.test: Likewise. * tests/acloca20.test: Likewise. * tests/acloca21.test: Likewise. * tests/acloca23.test: Likewise. * tests/aclocal-print-acdir.test: Likewise. * tests/ar-lib.test: Likewise. * tests/compile.test: Likewise. * tests/compile2.test: Likewise. * tests/compile3.test: Likewise. * tests/compile6.test: Likewise. * tests/instsh2.test: Likewise. * tests/instsh3.test: Likewise. * tests/mdate5.test: Likewise. * tests/mdate6.test: Likewise. * tests/missing3.test: Likewise. * tests/missing5.test: Likewise. * tests/mkinst3.test: Likewise. * tests/self-check-explicit-skips.test: Likewise. * tests/self-check-report.test: Likewise. * tests/self-check-sanity.test: Likewise. * tests/tap-driver-stderr.test: Likewise.