tests: prepare for parallel-tests support

* tests/Makefile.am (CLEANFILES): Calculate dynamically from TESTS.
* tests/init.sh (LOG): New variable.
* tests/detach-stopped: Use it.
* tests/net: Likewise.
* tests/qual_syscall: Likewise.
* tests/stat: Likewise.
* tests/strace-f: Likewise.
6 files changed