tests: posix: test_fs_file: Cut down on noise output

Remove bunch of TC_PRINT's with too verbose logging of test operations.
The only logging required is in case of failures, and improve that in
a couple of cases by using zassert_*(), and log errno.

In particular, this allows to reuse existing testcase functions to
create more complex testcases.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>