test(compiler-cli): compliance tests not always reporting test failure (#30597)

Currently the `@angular/compiler-cli` compliance tests sometimes do
not throw an exception if the expected output does not match the
generated JavaScript output. This can happen for the following cases:

1. Expected code includes character that is not part of known alphabet
    (e.g. `Δ` is still used in a new compliance test after rebasing a PR)
2. Expected code asserts that a string literal matches a string with
    escaped quotes. e.g. expects `const $var$ = "\"quoted\"";`)

PR Close #30597

packages/compiler-cli/test/compliance/mock_compiler_spec.ts

@@ -82,6 +82,34 @@ describe('mock_compiler', () => {
           result.source, 'name   \n\n   .  \n    length',
           'name length expression not found (whitespace)');
     });
+
+    it('should throw if the expected output contains unknown characters', () => {
+      const files = {
+        app: {
+          'test.ts': `ɵsayHello();`,
+        }
+      };
+
+      const result = compile(files, angularFiles);
+
+      expect(() => {
+        expectEmit(result.source, `ΔsayHello();`, 'Output does not match.');
+      }).toThrowError(/Invalid test, no token found for "Δ"/);
+    });
+
+    it('should be able to properly handle string literals with escaped quote', () => {
+      const files = {
+        app: {
+          'test.ts': String.raw `const identifier = "\"quoted\"";`,
+        }
+      };
+
+      const result = compile(files, angularFiles);
+
+      expect(() => {
+        expectEmit(result.source, String.raw `const $a$ = "\"quoted\"";`, 'Output does not match.');
+      }).not.toThrow();
+    });
   });
 
   it('should be able to skip untested regions (… and // ...)', () => {