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, 5 insertions, 1 deletions

diff --git a/railties/lib/rails/test_unit/reporter.rb b/railties/lib/rails/test_unit/reporter.rb
index bfdc9754d4..faf551f381 100644
--- a/railties/lib/rails/test_unit/reporter.rb
+++ b/railties/lib/rails/test_unit/reporter.rb
@@ -1,7 +1,11 @@
+require "active_support/core_ext/class/attribute"
 require "minitest"
 
 module Rails
   class TestUnitReporter < Minitest::StatisticsReporter
+    class_attribute :executable
+    self.executable = "bin/rails test"
+
     def report
       return if results.empty?
       io.puts
@@ -15,7 +19,7 @@ module Rails
       filtered_results.reject!(&:skipped?) unless options[:verbose]
       filtered_results.map do |result|
         location, line = result.method(result.name).source_location
-        "bin/rails test #{relative_path_for(location)}:#{line}"
+        "#{self.executable} #{relative_path_for(location)}:#{line}"
       end.join "\n"
     end