Fix Rails environment when running tests with Ruby

I frequently run tests with `ruby`, not with a runner like `rake` or
`rails`.  When running the test with just `ruby` the `RAILS_ENV`
environment variable did not get set to "test", and this would cause the
tests to fail (and even mutate the development database!)

This commit adds integration tests for running tests with just `ruby`
and ensures the environment gets defaulted to "test".  I also added a
test to ensure that passing an environment to `-e` actually works (and
fixed that case too).

An interesting / annoying thing is that Minitest picks up it's plugins
by asking RubyGems for a list of files:

  https://github.com/seattlerb/minitest/blob/ca6a71ca901016db09a5ad466b4adea4b52a504a/lib/minitest.rb#L92-L100

This means that RubyGems needs to somehow know about the file before it
can return it to Minitest.  Since we are not packaging Rails as a Gem
before running the integration tests on it (duh, why would you do
that?), RubyGems doesn't know about the file, so it can't tell Minitest,
so Minitest doesn't automatically require it.  This means I had to
manually require and insert the plugin in our integration test.  I've
left comments about that in the test as well.

Ugh.

1 files changed, 4 insertions, 2 deletions

diff --git a/railties/test/isolation/abstract_unit.rb b/railties/test/isolation/abstract_unit.rb
index 5b1c06d4e5..96c6f21395 100644
--- a/railties/test/isolation/abstract_unit.rb
+++ b/railties/test/isolation/abstract_unit.rb
@@ -358,10 +358,12 @@ module TestHelpers
     end
 
     def app_file(path, contents, mode = "w")
-      FileUtils.mkdir_p File.dirname("#{app_path}/#{path}")
-      File.open("#{app_path}/#{path}", mode) do |f|
+      file_name = "#{app_path}/#{path}"
+      FileUtils.mkdir_p File.dirname(file_name)
+      File.open(file_name, mode) do |f|
         f.puts contents
       end
+      file_name
     end
 
     def remove_file(path)