[Impeller] started asserting golden test runner has fatal impeller validations

[gaaclarke]

fixes https://github.com/flutter/flutter/issues/145017

This works by removing the conditional compilation for validation and turning them to be fatal in the test runner's main.

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[gaaclarke]

LGTM, but agree with expect_death if able.

I'll give that another try today.

[gaaclarke]

Yea looks like EXPECT_DEATH is unreliable in processes with more than one thread:

[ RUN      ] ValidationTest.IsFatal

[WARNING] ../../flutter/third_party/googletest/googletest/src/gtest-death-test.cc:1102:: Death tests use fork(), which is unsafe particularly in a threaded context. For this test, Google Test detected 6 threads. See https://github.com/google/googletest/blob/main/docs/advanced.md#death-tests-and-threads for more explanation and suggested solutions, especially if this is the last message you see before your test times out.
[FATAL:flutter/impeller/base/validation.cc(57)] Break on 'ImpellerValidationBreak' to inspect point of failure: 
Abort trap: 6

I looked to see if I could make sure the test happens first, presumably before the other threads are spawned, and it doesn't look like there is a way to make that the case reliably with googletest.