Due to the output format changes, revise the PosixUtils_test to fix
the failures.
Fix: 203494455
Test: atest libandroidfw_tests
Change-Id: I6e84f8011031162f9e46aa89ddf8d3f3b9863251
stdout and stderr are macros in musl, which causes build failures
when they are used as field names. Rename the fields to stdout_str
and stderr_str.
Test: builds
Change-Id: I04770d2d6e82ca8393308ab12a304838a80f0ae5