Tests UpdateIndexState and ExtraProgPath are weak/fragile

While working on https://github.com/haskell/cabal/pull/9609 (now merged), @tomsmeding discovered that the test cabal-testsuite/PackageTests/ExtraProgPath doesn't contain any `assertOutputContains`, but only requires a failure, which resulted in accepting a failure due to totally unrelated reasons. Also, the test cabal-testsuite/PackageTests/NewUpdate/UpdateIndexState uses port 8000, which sometimes makes it fail, because the port is popular and so often busy. I wonder how the other tests do that --- maybe it makes the most sense to choose a port similar, but different to what other tests choose.