setupScript writes a small shell script then immediately fork/execs
it through the runner. The existing write-tmp-then-rename pattern
prevents the userspace ETXTBSY race on a vanilla filesystem, but
overlayfs has an additional window where the kernel's view of
"who holds a writable fd to this inode" can lag the rename — and
the new container-based CI jobs hit it from time to time
("fork/exec ...: text file busy").
Switch the test temp dir to /dev/shm when available (tmpfs has
no overlay layering), falling back to t.TempDir() when /dev/shm
isn't usable. Production code path is unaffected; this is a
pure test helper change.
steve