doc: document that test_ functions may propagate errors (#10162)

When testing a function that can return an error, writing a test like
this allows you to more easily test the non-error flow by just
propagating any errors and letting them fail the test.

I didn't realize this was a feature at first, so I was writing tests
like:

```
fn test_atoi() {
	assert strconv.atoi('1') or { panic("atoi failed") } == 1
	assert strconv.atoi('one') or { panic("atoi failed") } == 1
}
```

doc/docs.md

@@ -3101,6 +3101,17 @@ You can also define special test functions in a test file:
 * `testsuite_begin` which will be run *before* all other test functions.
 * `testsuite_end` which will be run *after* all other test functions.
 
+If a test function has an error return type, any propagated errors will fail the test:
+
+```
+import strconv
+
+fn test_atoi() ? {
+	assert strconv.atoi('1') ? == 1
+	assert strconv.atoi('one') ? == 1 // test will fail
+}
+```
+
 #### Running tests
 
 To run test functions in an individual test file, use `v foo_test.v`.