maintcheck: avoid few spurious failures * tests/depmod-data.test: Use creative quoting to avoid spuriously triggering the `sc_tests_Exit_not_exit' maintainer check. Commit `v1.11-900-g3453b8e' attempted to fix it, but succeeded only partially. * tests/cond33.test: Fix header comments, not to reference obsolescent make variable `$(mkdir_p)'. * tests/cond4.test: Rewrite "$MAKE exp=..." as "exp=.. $MAKE -e", to please `sc_tests_overriding_macros_on_cmdline' * tests/cond19.test: Likewise. * tests/cond32.test: Likewise. * tests/add-missing.test: Use "AUTOMAKE_fails" instead of "$AUTOMAKE ... && Exit 1", to please `sc_tests_automake_fails'.