Investigate 'ch-run --join' clean failures on subsequent test suite runs.

The ipc_clean_p function from the ch-rub --join test fails during subsequent full scope test suite runs; /dev/shm appears to have leftover charliecloud crud.

 ✗ ch-run --join: one peer, direct launch
   (from function `ipc_clean_p' in file run/ch-run_join.bats, line 8,
    in test file run/ch-run_join.bats, line 83)
     `ipc_clean_p' failed
 ✗ ch-run --join: two peers, direct launch
   (from function `ipc_clean_p' in file run/ch-run_join.bats, line 8,
    in test file run/ch-run_join.bats, line 120)
     `ipc_clean_p' failed
 ✗ ch-run --join: three peers, direct launch
   (from function `ipc_clean_p' in file run/ch-run_join.bats, line 8,
    in test file run/ch-run_join.bats, line 159)
     `ipc_clean_p' failed
 ✗ ch-run --join: multiple peers, framework launch
   (from function `ipc_clean_p' in file run/ch-run_join.bats, line 8,
    in test file run/ch-run_join.bats, line 212)
     `ipc_clean_p' failed
...
jogas@ba224:testing $ find /dev/shm -maxdepth 1 -name '*ch-run*'
/dev/shm/ch-run_52
/dev/shm/sem.ch-run_52
/dev/shm/ch-run_51
/dev/shm/sem.ch-run_51
/dev/shm/ch-run_50
/dev/shm/sem.ch-run_50
/dev/shm/ch-run_49
/dev/shm/sem.ch-run_49
/dev/shm/ch-run_48
/dev/shm/sem.ch-run_48
/dev/shm/ch-run_47
/dev/shm/sem.ch-run_47
/dev/shm/ch-run_46
/dev/shm/sem.ch-run_46
/dev/shm/ch-run_45
/dev/shm/sem.ch-run_45
/dev/shm/ch-run_44
/dev/shm/sem.ch-run_44
/dev/shm/ch-run_43
/dev/shm/sem.ch-run_43