test: Add integration test for btrfs compression in repart

test: Add integration test for btrfs compression in repart

Add testcase_btrfs_compression() to verify that btrfs partitions with
Compression= and CopyFiles= directives work correctly.

The test verifies the fix for issue #39584, where mkfs.btrfs would fail
with "ERROR: --compression must be used with --rootdir" when repart
tried to create compressed btrfs filesystems.

The test creates a partition definition with Format=btrfs,
Compression=zstd, and CopyFiles=, then validates:

1. systemd-repart output shows "Rootdir from:" and "Compress:",
   confirming that the --rootdir code path is used
2. mkfs.btrfs is invoked with both --compress and --rootdir options
3. The file is successfully copied to the filesystem
4. Compression is actually applied (verified via compsize output
   containing "zstd")