[PTESTS] allows to redefine the topvel command

ptests/Makefile

ptests/ptests.ml

ptests/tests/cmd/README

ptests/tests/cmd/tests/ptests_config

ptests/tests/cmd/tests/test_config

ptests/tests/cmd/tests/without-test_config/README

ptests/tests/cmd/tests/without-test_config/config.i

ptests/tests/cmd/tests/without-test_config/empty.i

.../cmd/tests/without-test_config/oracle/config.0.res.oracle

.../cmd/tests/without-test_config/oracle/config.1.res.oracle

.../cmd/tests/without-test_config/oracle/config.2.res.oracle

.../cmd/tests/without-test_config/oracle/config.3.res.oracle

.../cmd/tests/without-test_config/oracle/config.4.res.oracle

...sts/cmd/tests/without-test_config/oracle/empty.res.oracle

ptests/tests/nothing/README

ptests/tests/nothing/tests/basic/oracle/empty.res.oracle

ptests/tests/nothing/tests/ptests_config