Fix and add new tests for controlling distribution of generated values

We have a number of tests showing the distribution of generated values, for example

```haskell
test_listDistribution :: Int -> ([()] -> Gen [[()]]) -> Int -> TestTree
test_listDistribution numRuns split n =
testProperty ("for a list of length " ++ show n) $
withMaxSuccess numRuns . forAll (split $ replicate n ()) $ \aSplit ->
label (show $ map length aSplit) True

test_multiSplitDistribution :: TestTree
test_multiSplitDistribution =
testGroup "distribution of values generated by"
[ testGroup "multiSplit1"
[ test_listDistribution 10000 (coerce $ multiSplit1 @()) 1
, test_listDistribution 10000 (coerce $ multiSplit1 @()) 2
, test_listDistribution 10000 (coerce $ multiSplit1 @()) 3
, test_listDistribution 10000 (coerce $ multiSplit1 @()) 4
, test_listDistribution 10000 (coerce $ multiSplit1 @()) 5
]
, testGroup "multiSplit0"
[ test_listDistribution 1000 (multiSplit0 0.1) 1
, test_listDistribution 1000 (multiSplit0 0.05) 2
, test_listDistribution 1000 (multiSplit0 0.01) 3
]
]
```

but we don't have any tests to ensure that the distribution is sensible, you have to eyeball it.

And when we do have such tests, they usefulness is quite limited:

```haskell
schnorrSecp256k1Prop :: PropertyT IO ()
schnorrSecp256k1Prop = do
testCase <- forAllWith ppShow genSchnorrCase
cover 5 "malformed verification key" . is (_ShouldError . _BadVerKey) $ testCase
cover 5 "malformed signature" . is (_ShouldError . _BadSignature) $ testCase
cover 5 "mismatch of signing key and verification key" . is (_Shouldn'tError . _WrongVerKey) $ testCase
cover 5 "mismatch of message and signature" . is (_Shouldn'tError . _WrongSignature) $ testCase
cover 5 "happy path" . is (_Shouldn'tError . _AllGood) $ testCase
runTestDataWith def testCase id VerifySchnorrSecp256k1Signature

ed25519Prop :: BuiltinSemanticsVariant DefaultFun -> PropertyT IO ()
ed25519Prop semvar = do
testCase <- forAllWith ppShow genEd25519Case
cover 5 "malformed verification key" . is (_ShouldError . _BadVerKey) $ testCase
cover 5 "malformed signature" . is (_ShouldError . _BadSignature) $ testCase
cover 5 "mismatch of signing key and verification key" . is (_Shouldn'tError . _WrongVerKey) $ testCase
cover 5 "mismatch of message and signature" . is (_Shouldn'tError . _WrongSignature) $ testCase
cover 5 "happy path" . is (_Shouldn'tError . _AllGood) $ testCase
runTestDataWith semvar testCase id VerifyEd25519Signature
```

Those `cover 5` should be `cover 15` or similar.

Find the tests containing test_listDistribution, test_multiSplitDistribution, schnorrSecp256k1Prop, and ed25519Prop, then read how generated-value distributions and cover requirements are currently checked. Add tests that verify the distributions are sensible and adjust the shown cover thresholds where appropriate; done means the checks no longer require eyeballing and the relevant test suite passes.

Written by the indexing model from the issue text.