Auto merge of #29296 - zazdxscf:compiletest_noargs_show_help, r=alexcrichton

instead of this panic:
```
thread '<main>' panicked at 'index out of bounds: the len is 1 but the
index is 1', src/libcollections/vec.rs:1110
```

It still panics, just like `-h` does, so it should be okay in this
regard.

diff --cc src/compiletest/compiletest.rs
Simple merge