Bug 857607 - Reduce log level when launching B2G desktop client with mozrunner. r=jgriffin, a=test-only

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extra : amend_source : 26aa240350f0dd4baf7f7c22195e4a8660f08e96
extra : transplant_source : %DB%C2%8AF2%97y%1F2%F8%04%F6%F5e%E2%81%1C%D0%C1%F7