These tests previously only ran on SerenityOS. They needed test input
location changes. The Stream tests also needed to explicitly set
SO_REUSEADDR for the tcp servers.
Let's replace this bool with an `enum class` in order to enhance
readability. This is done by repurposing `MappedFile`'s `OpenMode` into
a shared `enum` simply called `Mode`.
The internal reuse of FixedMemoryStream makes this straightforward.
There alread is one user of the new API, demonstrating the need for this
change beyond what I said out to use it for :^)
