Add support for generating multiple canvas test cases in the same file

This is done by specifying how a variant dimension should be generated:
either by putting all variants into the same file or each into their own
files. With multi-dimensional variants, each dimension can be
parameterized independently.

For reference tests, the test cases are arranged into a grid, with the
grid width being specified by the number of variants in the first
dimension. If more than two variant dimensions are specified, the grid
expands vertically, with all dimensions beyond the first one being
flattened down the HTML file. The default grid width can be overridden
by specifying the grid_width YAML param.

For testharness.js tests, individual test cases are all put into their
own test/async_test/promise_test.

As example, this CL migrates an existing multi-file reference and
testharness tests into single-file equivalent.

Fixed: 330885075
Change-Id: I619fbf0305b04787161d269158d3ed43990c78df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5393905


Reviewed-by: Andres Ricardo Perez <andresrperez@chromium.org>
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283511}