Convert FileCheck lowering tests into functional tests.

[paul-tqh-nguyen]

It was unclear how to move lower_random_select.mlir into a functional test.

I removed lowering test cases that were tested by functional tests.

[coveralls]

Pull Request Test Coverage Report for Build 1905605114

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 85.868%

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Totals
Change from base Build 1890935545:	0.0%
Covered Lines:	4563
Relevant Lines:	5314

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💛 - Coveralls