chore: Set screenshot threshold and kernel_size for all tests

[gadenbuie]

Following the logic in 301-bs-themes tests but with smaller kernel size

shinycoreci/inst/apps/301-bs-themes/tests/testthat/test-themes.R

Lines 82 to 84 in 33c0862

	# Allow for a 10% difference in the screenshot kernel
	# 3000 / (3x RGB channels * 100 * 100) = 3000 / 30000 = 10%
	app$expect_screenshot(threshold = 3000, kernel_size = 100)