tests: convert fallocate to use subtests

tests: convert fallocate to use subtests

Convert the fallocate test to use subtests pattern following the
waitpid test example. This allows testing multiple scenarios in a
single test run.

Subtests:
- alloc: Basic allocation test (original test)
- report-data-holes: Report holes before digging (data holes)
- dig-holes: Convert data holes to sparse holes with verbose output
- report-file-holes: Report holes after digging (file holes)

The test reuses the same image file across dig/report subtests to
efficiently test the complete workflow: report → dig → report.

Uses <testfile> placeholder instead of absolute paths in expected
output to ensure tests work from any directory.

Signed-off-by: Karel Zak <kzak@redhat.com>