Add redirects for integration-testing and headers (rust-lang#2092)

Author: jieyouxu

book.toml

@@ -54,3 +54,5 @@ warning-policy = "error"
 "/diagnostics/sessiondiagnostic.html" = "diagnostic-structs.html"
 "/diagnostics/diagnostic-codes.html" = "error-codes.html"
 "/miri.html" = "const-eval/interpret.html"
+"/tests/integration.html" = "ecosystem.html"
+"/tests/headers.html" = "directives.html"