Make generated canvas test parameters importable as Jinja templates

In the Canvas WPT test generator, the test body can have Jinja tags
referring to other test parameters. For instance:
  code: |
    @assert pixel 50,25 == {{ expected_color }};
  expected_color: 'rgba(0, 255, 0, {{ alpha }})'
  alpha: 1.0

Here, the `expected_color` parameter is first rendered, expanding the
`{{ alpha }}` tag. This result is then inserted into the `code`
template. This is useful to reuse values in multiple places. It however
doesn't allow reusing Jinja macros. Indeed, if `expected_color`
contained a `{% macro ... %}` tag, that tag would get lost as the
`expected_color` gets rendered (the macro itself is not part of the
rendering result).

To be able to reuse macros, we would need to use an `{% import ... %}`
tag. This CL makes this possible by registering the test parameters as
templates loadable by the Jinja environment. Thus, the following
becomes possible:

  macros: |
    {% macro calculate_color(...) %}
      ...
    {% endmacro %}
  code: |
    {% import 'macros' as m %}
    @assert pixel 50,25 == {{ m.calculate_color(...) }};
  expected: |
    {% import 'macros' as m %}
    size 100 50
    cr.set_source_rgba({{ m.calculate_color(...) }})

Bug: 40207206
Change-Id: Idff8495efffbcde1840256b5ac5bded1d7e6de55
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6204442
Reviewed-by: Andres Ricardo Perez <andresrperez@chromium.org>
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1420621}