[NFC] Cleanup the code used to run shell commands from tests
Previously we had 3 different method to run shell commands on the
target and 4 copy of code waiting until a given file appears on the
target device (used for syncronization). This CL merges these methods
to 1 run_platform_command and 1 wait_for_file_on_target functions
located in some utility classes.
Differential revision: http://reviews.llvm.org/D18789