Document test-only config settings. NFC (#23740)

Author: sbc100

tools/config.py

@@ -20,18 +20,10 @@
 # See parse_config_file below.
 EMSCRIPTEN_ROOT = __rootpath__
 NODE_JS = None
-NODE_JS_TEST = None
 BINARYEN_ROOT = None
-SPIDERMONKEY_ENGINE = None
-V8_ENGINE: Optional[List[str]] = None
-LLVM_ROOT = None
 LLVM_ADD_VERSION = None
 CLANG_ADD_VERSION = None
 CLOSURE_COMPILER = None
-JS_ENGINES: List[List[str]] = []
-WASMER = None
-WASMTIME = None
-WASM_ENGINES: List[List[str]] = []
 FROZEN_CACHE = None
 CACHE = None
 PORTS = None
@@ -40,6 +32,17 @@
 # Set by init()
 EM_CONFIG = None
 
+# Settings that are only used for testing.  emcc itself does not use
+# any of these.
+NODE_JS_TEST = None
+SPIDERMONKEY_ENGINE = None
+V8_ENGINE: Optional[List[str]] = None
+LLVM_ROOT = None
+JS_ENGINES: List[List[str]] = []
+WASMER = None
+WASMTIME = None
+WASM_ENGINES: List[List[str]] = []
+
 
 def listify(x):
   if x is None or type(x) is list: