SI-6210 Test case for already-fixed pattern matcher bug

The fix arrived in SI-6022 / #1100 / 2.10.0-M7.

Review by @adriaanm.

```
% RUNNER=scalac scala-hash v2.10.0-M7~26 -Xfatal-warnings test/files/pos/t6210.scala
[warn] v2.10.0-M7 failed, using closest available
[info] 07f942971a => /Users/jason/usr/scala-v2.10.0-M6-214-g07f9429
test/files/pos/t6210.scala:15: error: unreachable code
      case AConstBool(v: Boolean) => TBool()
                                          ^
one error found
% RUNNER=scalac scala-hash v2.10.0-M7~25 -Xfatal-warnings test/files/pos/t6210.scala
[info] v2.10.0-M7 => /Users/jason/usr/scala-v2.10.0-M6-219-gd1bdafa
```