SkQP: Relax metric tolerance to 600 for bitmap_subset_shader

gles_bitmap_subset_shader fail on VeriSilicon gc600 GPU.
Image mismatch: error metric = 544.000000

Bug: 133840004

Change-Id: Ib22d4e2b51e4d152bbf84ff7740df7db154573e9
Signed-off-by: Richard Liu <xuegang.liu@nxp.com>
1 file changed