tests: runtime: modified wait_for_file so it can wait for the file to have data inside so it can be used by the out_file test case.
tests: runtime: modified wait_for_file so it can wait for the file to
have data inside so it can be used by the out_file test case.

Signed-off-by: Leonardo Alminana <7610bae85f2b530654cc716772f1fe653373e892@calyptia.com>

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