Remove overlapping inputForm=LowForm tests (bp #1753)

[mergify]

This is an automated backport of pull request #1753 done by Mergify

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	both modified:   src/test/scala/firrtlTests/LoweringCompilersSpec.scala

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	both modified:   src/test/scala/firrtlTests/CustomTransformSpec.scala

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[seldridge]

This was manually cleaned up in a rebase to remove usages of verification statements. On 1.3.x, currently inputForm=LowForm or using optionalPrerequisites=Forms.LowFormOptimized will result in a transform running right before LegalizeAndReductionsTransform. This isn't necessarily correct (or wrong), but this PR is only trying to clean up tests to make future modifications more straightforward.