Set default color depth to 8 in nplb tests (#4729) #17997

Job	Run time
chromium_linux-x64 / initialize	1m 17s
linux / initialize	1m 13s
linux-evergreen / initialize	1m 19s
linux / docker-build-image	3m 51s
chromium_linux-x64 / docker-build-image	3m 55s
linux-evergreen / docker-build-image	4m 2s
linux / x64_devel	17m 13s
linux / x64_qa	16m 32s
linux / x64_gold	16m 39s
chromium_linux-x64 / x64_devel	11m 5s
chromium_linux-x64 / x64_qa	16m 0s
chromium_linux-x64 / x64_gold	14m 56s
linux-evergreen / x64_devel	8m 28s
linux-evergreen / x64_qa	8m 46s
linux-evergreen / x64_gold	8m 25s
linux-evergreen / ${{ matrix.name }}_tests_shard_${{ matrix.shard }}	0s
linux-evergreen / ${{ matrix.name }}_tests_passing	0s
linux-evergreen / ${{ matrix.name }}_tests_upload	0s
chromium_linux-x64 / ${{ matrix.name }}_tests_shard_${{ matrix.shard }}	0s
chromium_linux-x64 / ${{ matrix.name }}_tests_upload	0s
chromium_linux-x64 / ${{ matrix.name }}_tests_passing	0s
linux / x64_tests_shard_0	12m 20s
linux / x64_tests_shard_1	11m 55s
linux / x64_tests_shard_2	14m 57s
linux / x64_tests_shard_3	11m 10s
linux / x64_tests_shard_4	10m 34s
linux / x64_tests_shard_5	10m 53s
linux / x64_tests_shard_6	10m 23s
linux / x64_tests_shard_7	9m 32s
linux / x64_tests_shard_8	10m 57s
linux / x64_tests_shard_9	10m 36s
linux / x64_tests_passing	1s
linux / x64_tests_upload	1m 26s
	4h 8m 25s

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