perf test dso-data: Correctly free test file in read test

The DSO data read test opens a file but as dsos__exit is used the test
file isn't closed. This causes the subsequent subtests in don't fork
(-F) mode to fail as one more than expected file descriptor is open.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250318043151.137973-4-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>