How can I make sure a filesystem is ready for unmounting in a test script?

I'm using bats to test some bash scripts. In one of the test, I need to mount a generated iso disk image and make assertions on its content.

When I try to unmount the disk image right after the test, I get a Device or resource busy error unless I insert a sleep-time before the unmount operation.

The script looks like this:

setup() {
  load 'test_helper/bats-support/load'
  load 'test_helper/bats-assert/load'
}

teardown() {
  if [ -f "$BATS_TEST_TMPDIR"/mnt/my_file.txt ]; then
    sleep 1
    fusermount -u "$BATS_TEST_TMPDIR"/mnt
  fi
}

@test 'check iso content' {
  generate_iso "$BATS_TEST_TMPDIR"/my_iso.iso
  mkdir "$BATS_TEST_TMPDIR"/mnt
  fuseiso "$BATS_TEST_TMPDIR"/my_iso.iso "$BATS_TEST_TMPDIR"/mnt
  assert grep 'A required string' < "$BATS_TEST_TMPDIR"/mnt/my_file.txt
}

This somehow works but I'm not 100% happy with the arbitrary sleep-time that may or may not be sufficient for the unmount to be successful depending on how soon the file is closed.

I tried to kill the process accessing the mounted filesystem by using fuser -mk "$BATS_TEST_TMPDIR"/mnt instead of sleeping but this would eventually kill the process running the tests.

Is there any way I can avoid the arbitrary sleep-time? Can I ask the OS to wait for the file to be closed before proceeding? Any help would be greatly appreciated.

That was actually really easy, I just needed to close the file in the last grep (note the <&-):

assert grep 'A required string' <&- "$BATS_TEST_TMPDIR"/mnt/my_file.txt

teardown() {
  if [ -f "$BATS_TEST_TMPDIR"/mnt/my_file.txt ]; then
    fusermount -u "$BATS_TEST_TMPDIR"/mnt
  fi
}

@test 'check iso content' {
  generate_iso "$BATS_TEST_TMPDIR"/my_iso.iso
  mkdir "$BATS_TEST_TMPDIR"/mnt
  fuseiso "$BATS_TEST_TMPDIR"/my_iso.iso "$BATS_TEST_TMPDIR"/mnt
  assert grep 'A required string' <&- "$BATS_TEST_TMPDIR"/mnt/my_file.txt
}