[NFC] Give some JS optimizer tests nice names (emscripten-core#23161)

Author: kripken

test/js_optimizer/emitDCEGraph2-output.js renamed to test/js_optimizer/emitDCEGraph-dynCall-output.js

@@ -2892,10 +2892,10 @@ def test_extern_prepost(self):
     'AJSDCE': (['AJSDCE'],),
     'emitDCEGraph': (['emitDCEGraph', '--no-print'],),
     'emitDCEGraph-closure': (['emitDCEGraph', '--no-print', '--closure-friendly'], 'emitDCEGraph.js'),
-    'emitDCEGraph2': (['emitDCEGraph', '--no-print'],),
-    'emitDCEGraph3': (['emitDCEGraph', '--no-print'],),
-    'emitDCEGraph4': (['emitDCEGraph', '--no-print'],),
-    'emitDCEGraph5': (['emitDCEGraph', '--no-print'],),
+    'emitDCEGraph-dynCall': (['emitDCEGraph', '--no-print'],),
+    'emitDCEGraph-eval': (['emitDCEGraph', '--no-print'],),
+    'emitDCEGraph-sig': (['emitDCEGraph', '--no-print'],),
+    'emitDCEGraph-prefixing': (['emitDCEGraph', '--no-print'],),
     'emitDCEGraph-scopes': (['emitDCEGraph', '--no-print'],),
     'minimal-runtime-applyDCEGraphRemovals': (['applyDCEGraphRemovals'],),
     'applyDCEGraphRemovals': (['applyDCEGraphRemovals'],),