test_nightly (and other tests) should cleanup on success

[leftwo]

Running tools/test_nightly.sh puts a fair strain on /var/tmp

hu Feb 20 07:35:05 UTC 2025 Begin pretest initial fill
./tools/test_live_repair.sh: line 118: 319466 Aborted                 (core dumped) "$crucible_test" fill --dsc 127.0.0.1:9998 -q -g "$gen" --skip-verify >> "$test_log" 2>&1
Failed on initial fill, check /tmp/test_live_repair.log

 #####   ######  #####   #         ##     ####   ######          #####   ######
 #    #  #       #    #  #        #  #   #    #  #               #    #  #
 #    #  #####   #    #  #       #    #  #       #####           #    #  #####
 #####   #       #####   #       ######  #       #               #####   #
 #   #   #       #       #       #    #  #    #  #               #   #   #
 #    #  ######  #       ######  #    #   ####   ###### #######  #    #  ######

mkdir: cannot create directory ‘/var/tmp/test_replace_special’: No space left on device
./tools/test_replace_special.sh: line 68: echo: write error: No space left on device
./tools/test_replace_special.sh: line 69: echo: write error: No space left on device
starting Thu Feb 20 07:35:40 UTC 2025
tee: /tmp/test_replace_special_summary.log: No space left on device
Tail /tmp/test_replace_special.log for test output
thread 'main' panicked at std/src/io/stdio.rs:1123:9:
failed printing to stdout: No space left on device (os error 28)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
./tools/test_replace_special.sh: line 84: 319584 Aborted                 (core dumped) ${dsc} create --cleanup --region-dir "$REGION_ROOT" --region-count "$region_count" --ds-bin "$downstairs" --extent-count 400 --block-size 4096 >> "$test_log"
Failed to create downstairs regions

The tests themselves should clean up if they finish with success.
The nightly test script should also have an option on where to put the regions. Many tests support this (not all).
The tests should be updated to inherit a global REGION_DIR and nightly should take one from the caller.