make it possible to customize the executable inside rereun snippets.

In the Rails repository we use a `bin/test` executable to run our tests.
However the rerun snippets still included `bin/rails test`:

BEFORE:
```
Failed tests:

bin/rails test test/cases/adapters/postgresql/schema_test.rb:91
```

AFTER:
```
Failed tests:

bin/test test/cases/adapters/postgresql/schema_test.rb:91
```

1 files changed, 2 insertions, 0 deletions

diff --git a/tools/test.rb b/tools/test.rb
index 50cce2081a..70f295b554 100644
--- a/tools/test.rb
+++ b/tools/test.rb
@@ -8,3 +8,5 @@ module Rails
     @root ||= Pathname.new(COMPONENT_ROOT)
   end
 end
+
+Rails::TestUnitReporter.executable = "bin/test"