smoketest: write run_in_vm's log as the command runs

So a long command (install.sh takes an hour or more) can be followed with
tail -f; run_in_vm prints the log's path when it starts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/smoketest.bats

@@ -131,6 +131,12 @@ upstream_main() {
   [ -f "$SMOKETEST_RESULTS/02-second.log" ]
 }
 
+@test "run_in_vm writes its log while the script runs" {
+  run_in_vm slow 'echo early; cat "$SMOKETEST_RESULTS/01-slow.log" > "$HOME/seen"'
+  [ "$STATUS" -eq 0 ]
+  [ "$(cat "$HOME/seen")" = early ]
+}
+
 @test "cd_update passes its variables to update" {
   mkdir -p "$HOME/.local/bin"
   printf '%s\n' '#!/bin/bash' 'echo "force=$UPDATE_FORCE"' > "$HOME/.local/bin/update"