Bug 1907637 - test(webgpu): set `--timeout-multiplier=5.0` for CTS r=webgpu-reviewers,teoxoy

Running the following command with Nushell (`nu`) on
[`gpuweb/cts`:0ba03b43]:

[`gpuweb/cts`:0ba03b43]: 0ba03b439c/src/webgpu/listing_meta.json

```
open src\webgpu\listing_meta.json
	| reject _end _comment
	| transpose
	| rename test data
	| update data {
		$in.subcaseMS
			| into float
			| into int
			| into duration --unit ms
	}
	| rename --column { data: subcaseMS }
	| sort-by subcaseMS
	| insert violates_the_law { $in.subcaseMS > 30sec }
	| filter { $in.violates_the_law }
	| reject violates_the_law
	| last
```

…which results in this output:

```
╭───────────┬───────────────────────────────────────────────────────────────╮
│ test      │ webgpu:shader,validation,expression,call,builtin,max:values:* │
│ subcaseMS │ 4min 20sec 241ms                                              │
╰───────────┴───────────────────────────────────────────────────────────────╯
```

…we can determine that Chromium's test runs of WebGPU CTS expect their
longest-running test to take approximately 4.3 minutes. Rounding up,
this approximately 5 times the base long timeout of a WPT test for
Firefox, by default. So, apply a 5x test timeout to Firefox's WebGPU CTS
tests, in hopes that we can avoid the massive number of `TIMEOUT`s we've
historically run into.

Differential Revision: https://phabricator.services.mozilla.com/D216439

taskcluster/kinds/test/web-platform.yml

@@ -295,6 +295,7 @@ web-platform-tests-webgpu:
                     - --tag=webgpu
                     - --exclude-tag=canvas
                     - --exclude-tag=webcodecs
+                    - --timeout-multiplier=5.0
                     - --setpref=gfx.webgpu.ignore-blocklist=true
                 default:
                     - --test-type=testharness
@@ -304,6 +305,7 @@ web-platform-tests-webgpu:
                     - --tag=webgpu
                     - --exclude-tag=canvas
                     - --exclude-tag=webcodecs
+                    - --timeout-multiplier=5.0
     worker-type:
         by-test-platform:
             .*wayland.*: t-linux-wayland-vm
@@ -350,6 +352,7 @@ web-platform-tests-webgpu-backlog:
                     - --skip-implementation-status=implementing
                     - --skip-implementation-status=not-implementing
                     - --backlog
+                    - --timeout-multiplier=5.0
                     - --setpref=gfx.webgpu.ignore-blocklist=true
                 default:
                     - --test-type=testharness
@@ -360,6 +363,7 @@ web-platform-tests-webgpu-backlog:
                     - --skip-implementation-status=implementing
                     - --skip-implementation-status=not-implementing
                     - --backlog
+                    - --timeout-multiplier=5.0
     worker-type:
         by-test-platform:
             .*wayland.*: t-linux-wayland-vm