generic/126: run it inside its own directory

To: fstests@vger.kernel.org
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>

---
Changes in v3:
- Add comment explaining the weird setup
- Move all of the cleanup into _cleanup()
- rm -rf $testdir before doing mkdir
- Link to v2: https://lore.kernel.org/r/20250219-generic126-v2-1-e7537f6c9607@kernel.org

Changes in v2:
- add commands to _cleanup() to clean up test directory (Thanks Darrick!)
- Link to v1: https://lore.kernel.org/r/20250219-generic126-v1-1-018e4e60c811@kernel.org

--- b4-submit-tracking ---
# This section is used internally by b4 prep for tracking purposes.
{
  "series": {
    "revision": 3,
    "change-id": "20250219-generic126-f81b3022c4b7",
    "prefixes": [
      "fstests"
    ],
    "history": {
      "v1": [
        "20250219-generic126-v1-1-018e4e60c811@kernel.org"
      ],
      "v2": [
        "20250219-generic126-v2-1-e7537f6c9607@kernel.org"
      ]
    }
  }
}