[Backend Tester] Clean up a few test issues

[GregoryComer]

There are a few broken tests that need cleaning up. Some are failing due to missing portable kernels. These tests are now skipped if any unsupported portable ops remain post-delegation. I also fixed a few other small issues and bumped the element-wise tolerance to reduce false positives. SNR should hopefully catch most blatant correctness issues. The fp16 and quantized tests can generate occasional high element-wise error but still have decent SNR (~60+).

[digantdesai]

If we are adding a Portable flow, how would these show up there? Some PTE_FAIL?

[GregoryComer]

Fair question. They won't be reported as failures. I think that's mostly fine, since the main goal is to validate backends. That being said, we could only skip these tests for flows that are expected to delegate - though backend op libs might make it trickier. I'll make this change.

[github-actions]

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