Fix misleading stray PTRACE_EVENT_EXEC diagnostics

Fix misleading stray PTRACE_EVENT_EXEC diagnostics

If current_tcp is to be switched, entering(current_tcp) is irrelevant.

* strace.c (maybe_switch_tcbs): Update comment.
(dispatch_event) <case TE_STOP_BEFORE_EXECVE>: Invoke
maybe_switch_current_tcp early, skip the check for a stray
PTRACE_EVENT_EXEC if current_tcp was switched.
* tests/maybe_switch_current_tcp.c: New file.
* tests/pure_executables.list: Add maybe_switch_current_tcp.
* tests/.gitignore: Likewise.
* tests/gen_tests.in (maybe_switch_current_tcp): New entry.
* tests/Makefile.am (maybe_switch_current_tcp_LDADD): New variable.