tests first
fix 'test.makeCacheMatrix.returnsSomething':
- method name was wrong for the `check` function
- the comparison was being vectorized and checking each element of the list for null instead of the value as a whole, using identical instead
- make the `test.cacheMatrix.newDataInvalidatesCache` test independent of `cacheSolve`
- add a test to show that `setInverse` causes caching of the value
use `is.null` instead of `identical` in the tests, reads better
passes all tests, needs comments
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