fix(pgtest): use t.Fatalf instead of t.Errorf for missing CL_DATABASE_URL (#21956)

pgtest.NewSqlxDB used t.Errorf (non-fatal) when CL_DATABASE_URL was
missing, then returned nil. This caused SIGSEGV panics in downstream
tests when they called methods on the nil *sqlx.DB.

Changing to t.Fatalf stops test execution immediately, preventing the
nil DB from propagating.

Fixes: CORE-2371

Author: Fletch153

core/internal/testutils/pgtest/pgtest.go

@@ -18,8 +18,7 @@ func NewSqlxDB(t testing.TB) *sqlx.DB {
 	testutils.SkipShortDB(t)
 	dbURL := string(env.DatabaseURL.Get())
 	if dbURL == "" {
-		t.Errorf("you must provide a CL_DATABASE_URL environment variable")
-		return nil
+		t.Fatalf("you must provide a CL_DATABASE_URL environment variable")
 	}
 	return sqltest.NewDB(t, dbURL)
 }