docs: point the test database setting at the right database

Same correction as the one on the helper itself: "isolated" here means
isolated from anything you care about, not isolated from DATABASE_URL.
Filling this in with a second database name is a plausible reading that
fails the suite in a way that does not point back here.

.env.example

@@ -11,8 +11,16 @@ POSTGRES_DB=nestri
 # For anything run outside a container — `bun dev`, `bun run db:migrate`.
 DATABASE_URL=postgres://postgres:postgres@localhost:5432/nestri
 
-# Isolated database for tests. Required by DB-backed tests; unset tests fail.
-TEST_DATABASE_URL=
+# The database the tests run against. Required — DB-backed tests refuse to run
+# rather than fall back to a database nobody named.
+#
+# **Point it at the same database as DATABASE_URL above.** "Isolated" means
+# isolated from anything you care about, not isolated from DATABASE_URL: route
+# tests reach the database through the app and core tests reach it directly, so
+# two different values put the fixtures in one database and the assertions in
+# the other. That fails around forty tests, in neither half's own code, with
+# nothing in the output pointing at this line.
+TEST_DATABASE_URL=postgres://postgres:postgres@localhost:5432/nestri
 
 # The issuer's public URL — the address a token's `iss` claim will carry.
 AUTH_ISSUER_URL=http://localhost:1337