Need help on M Query

You aren't too far off but I think I'd write it like this:

= Table.AddColumn(
    #"Changed Type1",
    "Custom",
    (t) => List.Last(
               List.Select(
                   TableB[LookupValue],
                   each Text.Contains(t[Test], _)
               )
           ) ?? "Not Found",
    type text
)

 The "??" part indicates what to return if the preceding expression is null.

Compute them separately and use if/then logic to return the text you want.

(t) =>
    [
        Partial = List.Last(List.Select(TableB[LookupValue], each Text.Contains(t[Test], _))),
        Exact   = List.Last(List.Select(TableB[LookupValue], each t[Test] = _)),
        Text    =
            if Exact <> null then "Exact match " & Exact
            else if Partial <> null then "Partial match " & Partial
            else "No match"
    ][Text]