lib: fix coverage reporting

Taking the source code of a function and running it in another
context does not play well with coverage instrumentation.
For now, do the simple thing and just write the source code as
a string literal.

Fixes: nodejs#19912
Refs: nodejs#19524

lib/internal/util.js

@@ -390,17 +390,16 @@ function isInsideNodeModules() {
     // Use `runInNewContext()` to get something tamper-proof and
     // side-effect-free. Since this is currently only used for a deprecated API,
     // the perf implications should be okay.
-    getStructuredStack = runInNewContext('(' + function() {
+    getStructuredStack = runInNewContext(`function() {
       Error.prepareStackTrace = function(err, trace) {
         err.stack = trace;
       };
       Error.stackTraceLimit = Infinity;
 
       return function structuredStack() {
-        // eslint-disable-next-line no-restricted-syntax
         return new Error().stack;
       };
-    } + ')()', {}, { filename: 'structured-stack' });
+    })()`, {}, { filename: 'structured-stack' });
   }
 
   const stack = getStructuredStack();