Generic matrix testing -- left and right kernel

[mkoeppe]

• Rebased version of Generic matrix testing -- left and right kernel #7991

Fixes #7991.

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• The title is concise and informative.
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• I have created tests covering the changes.
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[orlitzky]

This predates our random methods actually being random. I think it would be more transparent/direct to add TESTS: to each of the kernel methods that constructs a random matrix (random size, random base ring, random entries) and then checks these properties.