update memory leak test for node v12.4

Looking at heapUsed is not very reliable, because v8 in Node v12.4 just
uses more memory in general.  It's better to look at detached contexts,
which also does not require calling gc() to test.

test/avoid-memory-leak.js

@@ -1,25 +1,41 @@
 var importFresh = require('import-fresh');
 var t = require('tap')
+var v8
+try {
+  v8 = require('v8')
+} catch (er) {}
+
+var previousHeapStats
+
+function checkHeap (t) {
+  var v8stats = v8 ? v8.getHeapStatistics() : {}
+  var stats = process.memoryUsage()
+  if (typeof v8stats.number_of_detached_contexts === 'number')
+    t.equal(v8stats.number_of_detached_contexts, 0, 'no detached contexts')
+  else {
+    const memoryUsage = stats.heapUsed - previousHeapStats.heapUsed
+    const memoryUsageMB = Math.round(memoryUsage / Math.pow(1024, 2))
+    t.ok(memoryUsageMB < 2, 'expect less than 2MB difference, '
+      + memoryUsageMB + 'MB difference found.');
+  }
+}
 
 t.test('no memory leak when loading multiple times', function(t) {
-    t.plan(1);
-    importFresh(process.cwd() + '/graceful-fs.js') // node 0.10-5 were getting: Cannot find module '../'
-    const mbUsedBefore = process.memoryUsage().heapUsed / Math.pow(1024, 2);
-    // simulate project with 4000 tests
-    var i = 0;
-    function importFreshGracefulFs() {
-        importFresh(process.cwd() + '/graceful-fs.js');
-        if (i < 4000) {
-            i++;
-            process.nextTick(() => importFreshGracefulFs());
-        } else {
-            global.gc();
-            const mbUsedAfter = process.memoryUsage().heapUsed / Math.pow(1024, 2);
-            // We expect less than a 2 MB difference
-            const memoryUsageMB = Math.round(mbUsedAfter - mbUsedBefore);
-            t.ok(memoryUsageMB < 2, 'We expect less than 2MB difference, but ' + memoryUsageMB + 'MB is still claimed.');
-            t.end();
-        }
+  t.plan(1);
+  importFresh(process.cwd() + '/graceful-fs.js') // node 0.10-5 were getting: Cannot find module '../'
+  previousHeapStats = process.memoryUsage()
+  // simulate project with 4000 tests
+  var i = 0;
+  function importFreshGracefulFs() {
+    importFresh(process.cwd() + '/graceful-fs.js');
+    if (i < 4000) {
+      i++;
+      process.nextTick(() => importFreshGracefulFs());
+    } else {
+      global.gc()
+      checkHeap(t);
+      t.end();
     }
-    importFreshGracefulFs();
+  }
+  importFreshGracefulFs();
 })