Don't synthesize invalid errors in CommandBufferSharedTest

See the following error:
https://luci-milo.appspot.com/ui/inv/build-8752488687744028897/test-results?q=CommandBufferSharedTest.TestConsistency&sortby=&groupby=

It is UB to cast to an enum if it is outside the enum's range. An enum's
range is pretty complex and messy[*], but since gpu::error::Error does not
appear to be made for arbitrary enum values, let's just use real values
in the test.

[*] If you write `enum class` or `enum Error : int`, the range is the
full range of the underlying type. If you write just a plain enum, the
range is the minimal-width hypothetical integer that can hold all the
enum values. Since this enum goes 0 to 8, its range is actually 0..15,
for a hypothetically uint4_t. It's bizarre.

Bug: 40942951
Change-Id: Ic3beee912e7394256f591fc40855b4004b86434e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402196


Auto-Submit: David Benjamin <davidben@chromium.org>
Reviewed-by: Victor Miura <vmiura@chromium.org>
Commit-Queue: Victor Miura <vmiura@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279906}