report,ui: name the tee log by flow kind

BuildTarget::source_only carried two states for what is a three-way
distinction now that pkh test exists: replace it with a TargetKind
(Source/Binary/Test) and derive the tee log prefix (build-/deb-/test-)
from it, so test runs get their own log family instead of borrowing
the deb one.

src/ui/deb.rs

@@ -272,7 +272,11 @@ impl crate::report::BuildView for DebUi {
             self.shared.top.set_prefix(target.display.clone());
         }
         if target.tee_log {
-            let kind = if target.source_only { "build" } else { "deb" };
+            let kind = match target.kind {
+                crate::report::TargetKind::Source => "build",
+                crate::report::TargetKind::Binary => "deb",
+                crate::report::TargetKind::Test => "test",
+            };
             self.open_log(
                 kind,
                 target.package,