test_runner: fix escaping in snapshot tests

Snapshotted values are escaped after serialization.
This happens when serializing a value for comparison
when snapshots already exist, and also when updating them.
That is, snapshots are escaped in the internal storage,
but when written to disk, one "level" of escaping is removed.
That escaping is never added back when reading the snapshots back.
This makes even the simplest test trying to serialize a string
with an escape code in it fail, like the one I added here.

lib/internal/test_runner/snapshot.js

@@ -146,7 +146,9 @@ class SnapshotManager {
         );
       }
 
-      this.snapshots = context.exports;
+      for (const key in context.exports) {
+        this.snapshots[key] = templateEscape(context.exports[key]);
+      }
       this.loaded = true;
     } catch (err) {
       let msg = `Cannot read snapshot file '${this.snapshotFile}.'`;

test/fixtures/test-runner/snapshots/unit.js

@@ -22,3 +22,7 @@ test('`${foo}`', async (t) => {
   const options = { serializers: [() => { return '***'; }]};
   t.assert.snapshot('snapshotted string', options);
 });
+
+test('escapes in `\\${foo}`\n', async (t) => {
+  t.assert.snapshot('`\\${foo}`\n');
+});

test/parallel/test-runner-snapshot-tests.js

@@ -296,9 +296,9 @@ test('t.assert.snapshot()', async (t) => {
 
     t.assert.strictEqual(child.code, 1);
     t.assert.strictEqual(child.signal, null);
-    t.assert.match(child.stdout, /# tests 3/);
+    t.assert.match(child.stdout, /# tests 4/);
     t.assert.match(child.stdout, /# pass 0/);
-    t.assert.match(child.stdout, /# fail 3/);
+    t.assert.match(child.stdout, /# fail 4/);
     t.assert.match(child.stdout, /Missing snapshots/);
   });
 
@@ -311,8 +311,8 @@ test('t.assert.snapshot()', async (t) => {
 
     t.assert.strictEqual(child.code, 0);
     t.assert.strictEqual(child.signal, null);
-    t.assert.match(child.stdout, /tests 3/);
-    t.assert.match(child.stdout, /pass 3/);
+    t.assert.match(child.stdout, /tests 4/);
+    t.assert.match(child.stdout, /pass 4/);
     t.assert.match(child.stdout, /fail 0/);
   });
 
@@ -325,8 +325,8 @@ test('t.assert.snapshot()', async (t) => {
 
     t.assert.strictEqual(child.code, 0);
     t.assert.strictEqual(child.signal, null);
-    t.assert.match(child.stdout, /tests 3/);
-    t.assert.match(child.stdout, /pass 3/);
+    t.assert.match(child.stdout, /tests 4/);
+    t.assert.match(child.stdout, /pass 4/);
     t.assert.match(child.stdout, /fail 0/);
   });
 });