prefer hyphens in test files named after issue numbers

We have a lot of tests with filenames honoring particular issues by
number. Typically, these are called issue-${issue_no}.rs (note the
hyphen):

```
$ find . -regextype posix-egrep -regex '.*/issue-[0-9]*.rs' | wc
   1289    1289   35935
```

We also had a much smaller number of files that are like this, but don't
have a hyphen in between the substring `issue` and the number:

```
$ find . -regextype posix-egrep -regex '.*/issue[0-9]*.rs'
./debuginfo/issue14411.rs
./debuginfo/issue12886.rs
./debuginfo/issue13213.rs
./debuginfo/issue22656.rs
./debuginfo/issue7712.rs
./compile-fail/issue32829.rs
./run-pass/issue24353.rs
./run-pass/issue34796.rs
./run-pass/issue18173.rs
./run-pass/issue22346.rs
./run-pass/auxiliary/issue13507.rs
./run-pass/issue26127.rs
./run-pass/issue22008.rs
./run-pass/issue34569.rs
./run-pass/issue29927.rs
./run-pass/issue36260.rs
```

Some would argue that the inconsistency is æsthetically displeasing,
hence this trivial patch. (Note that run-pass/auxiliary/issue13507.rs
has an excuse; it's `use`d in run-pass/issue-13507-2.rs; the matter of
there being two different compile-fail tests with different name
conventions for issue #32829 is also neglected here for the sake of
keeping this trivial cleanup patch as trivial as possible for ease of
review.)

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