chore: improve tests without assertions

[jorgsowa]

Test suite warns that the test doesn't have any assertions, so let's add it.

• test/snapshot/namespace.test.js - filled 3 empty todo tests. Two of them were redundant. There were not enough tests for the use statement.
• test/snapshot/string.test.js - applied assertions to the test. The test only tested parsing, without checking the proper structure.
• test/ast.test.js - added assertion

[czosel]

👍