Using io.open instead of built-in open for unit test. (#857)
Using io.open instead of built-in open for unit test. (#857)

PiperOrigin-RevId: 230830063
google/openhtf,google/openhtf,jettisonjoe/openhtf,jettisonjoe/openhtf,jettisonjoe/openhtf,google/openhtf,jettisonjoe/openhtf,google/openhtf