fix(testing): assertThrowsAsync always reporting Error instead of actual error class

[lowlighter]

Closes #967

As assertThrowsAsync has an implementation drift from assertThrows which result in hiding the actual typing of errors in result messages when using custom errors:

class CustomError extends Error {}
Deno.test("test", async () => void await assertThrowsAsync(() => Promise.reject(new CustomError()), TypeError))

Before (making test harder to debug):

AssertionError: Expected error to be instance of "TypeError", but got "Error".

After:

AssertionError: Expected error to be instance of "TypeError", but got "CustomError ".

---

References:

AssertThrows:
https://github.com/denoland/deno_std/blob/e4c6419c466335eb697df38a387a2def06e6f3c5/testing/asserts.ts#L598

AssertThrowsAsync:
https://github.com/denoland/deno_std/blob/e4c6419c466335eb697df38a387a2def06e6f3c5/testing/asserts.ts#L644

[bartlomieju]

@lowlighter the fix looks good, could you exercise this change in testing/asserts_test.ts and check that the returned message is as expected?

[kt3k]

👍

[jsejcksn]

@kt3k (I know this is already merged, but...)

Is there a convention for grammar in error messages?

For example, things like:

• terminating punctuation vs no terminating punctuation:
  • "File not found."
  • "File not found"
• conventional verbs (i.e. got/saw/was/etc.)
  • "Expected x but saw y"
  • "Expected x but got y"
• etc.

[kt3k]

@jsejcksn I don't think we have an explicit rule about it. But I think we should have one. Suggestions are welcome

[kt3k]

@lowlighter Thank you for your contribution! LGTM