Verify all unit test whether the formatted code is valid

[nojaf]

In light of #841, we should check whether each unit test is producing valid F# code.
We currently only compare afterwards if each test is matching the expected string.
This is not enough, bugs can still slip into this if the result is not valid.

//cc @knocte @Bobface

[knocte]

And in light of #868 maybe it should check for warnings too?

[nojaf]

Yes, seems like a good idea.

[knocte]

The commit 17fa4cb doesn't seem to fix this issue, @nojaf typo in commit message?

[nojaf]

Indeed, I tried to remove the link in (#1015) but the commit message still triggered closing this.