Add some testcases for recent rustfix update

[hellow554]

changelog: none

This adds a testcase for a bugfix that has been fixed by https://github.com/rust-lang/rustfix/tree/v0.6.1

rustfix is pulled in by compiletest_rs. So to test that the correct rustfix version is used, I added one (and a half) testcase.

I tried to add a testcase for #8734 as well, but interesting enough the rustfix is wrong:

 fn issue8734() {
     let _ = [0u8, 1, 2, 3]
         .into_iter()
-        .and_then(|n| match n {
+        .flat_map(|n| match n {
+            1 => [n
+                .saturating_add(1)
             1 => [n
                 .saturating_add(1)
                 .saturating_add(1)
                 .saturating_add(1)
                 .saturating_add(1)
                 .saturating_add(1)
                 .saturating_add(1)
                 .saturating_add(1)
                 .saturating_add(1)],
             n => [n],
         });
 }

this needs some investigation and then this testcase needs to be enabled by commenting it out

closes #8878
related to #8734

[rust-highfive]

r? @Manishearth

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[Manishearth]

@bors r+

[bors]

📌 Commit 04297de has been approved by Manishearth

[bors]

⌛ Testing commit 04297de with merge 67a0891...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing 67a0891 to master...