use random file name for RoundTripFileWithNoSheetCols test

[benedictjohannes]

In my test, RoundTripFileWithNoSheetCols fails with error e"OpenFile: open testdocs/after_write.xlsx: no such file or directory" This is most likely due to the parallel test runs called by csRunO that try to write and remove the files concurrently. Failed error attached

By choosing a random filename, each test create/write and removes its own file, allowing this test to pass as expected.

[ghost]

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