Correct stale comments in the deb install test

The lead line above start_unit described the non-blocking start the
function replaced, contradicting both the body and the paragraph that
follows; drop it. The build banner still named the Debian 12 builder
image the suite stopped using; name the shared container script and the
file that sets its image instead.

The header and the install check also called /etc/fips/fips.yaml a
conffile. It is not one: the package ships fips.yaml.example and
postinst seeds fips.yaml from it when absent, while the declared
conffiles are hosts and fips.nft. Name fips.nft as the conffile the
suite checks, and say the check sees a postinst-seeded file. Comment and
message text only; no check changes.

testing/deb-install/test.sh

@@ -15,7 +15,8 @@
 #
 # This is the most thorough test surface — it exercises:
 #   - cargo deb packaging (binary stripping, dependency declaration)
-#   - dpkg conffile placement (/etc/fips/fips.yaml)
+#   - dpkg conffile placement (/etc/fips/fips.nft) and postinst seeding
+#     of /etc/fips/fips.yaml
 #   - postinst maintainer scripts (systemd unit enablement,
 #     fips-dns.service running fips-dns-setup)
 #   - postrm purge (removing the DNS routing fips-dns-setup wrote)
@@ -141,8 +142,6 @@ wait_for_systemd() {
     return 1
 }
 
-# Start a unit without waiting for its start job to finish.
-#
 # Start a unit and wait for its start job, under a bound.
 #
 # Blocking is the right default and the call returning is what synchronises the
@@ -194,8 +193,9 @@ container_systemd_version() {
 }
 
 # ─────────────────────────────────────────────────────────────────────
-# Build the .deb once in a Debian 12 cargo-deb builder image (cached
-# between runs). Output cached at testing/deb-install/.cache/deb/.
+# Build the .deb once through packaging/debian/build-deb-container.sh,
+# whose image is set in packaging/build-floor.env (cached between
+# runs). Output cached at testing/deb-install/.cache/deb/.
 # Rebuilt if any source/Cargo/packaging file is newer than the cached
 # .deb, or if the .deb is missing.
 # ─────────────────────────────────────────────────────────────────────
@@ -542,9 +542,9 @@ DOCKERFILE
         fail "/usr/bin/fips-gateway missing"
     fi
     if docker exec "$name" test -f /etc/fips/fips.yaml; then
-        pass "/etc/fips/fips.yaml conffile installed"
+        pass "/etc/fips/fips.yaml seeded by postinst"
     else
-        fail "/etc/fips/fips.yaml conffile missing"
+        fail "/etc/fips/fips.yaml missing"
     fi
 
     # Verify fips.service is enabled (postinst enables but does not
@@ -670,7 +670,7 @@ DOCKERFILE
     fi
 
     # Get the daemon's npub via fipsctl. Works for both ephemeral
-    # and persistent identity (no need to override the conffile).
+    # and persistent identity (no need to override /etc/fips/fips.yaml).
     local npub
     npub=$(docker exec "$name" fipsctl show status 2>/dev/null \
         | grep -oE 'npub1[a-z0-9]+' | head -1)