Improve the testing guide [ci skip]

Improve the "Testing Rails Applications" guide adding a section for
testing helpers and a little note about associations with ActiveRecord
and YAML fixtures.

Also add a link to Fixture Builder ; yet another replacement for
fixtures such as Machinist or FactoryGirl.
Remove conditional, since results are always an instance of `ActiveRecord::Result`
Fix simple_format output example ending tag
Merge pull request #11383 from mattdbridges/simple-format-doc-fix

Fix simple_format output example ending tag
pass arel to select_all rather than depend on method_missing
fix visibility of the relation construction methods
Remove unnecessary string interpolation

Removed from controller template of scaffold generator.
Merge pull request #11385 from MarceloCajueiro/remove_string_interpolation

Remove unnecessary string interpolation
Update the filestore documentation for clear
[ci skip]
Add documentation for #clear on certain Store classes
[ci skip
Add documentation for FileStore#increment and #decrement
[ci skip]
Avoid creating an extra Range instance
Speedup String#to

Benchmark:
1000000.times { str.to(30) }
          user     system      total        real
old   0.490000   0.110000   0.600000 (  0.607374)
new   0.390000   0.000000   0.390000 (  0.387306)
Speedup AS::Inflector.deconstantize

Benchmark:
          user     system      total        real
old   0.740000   0.000000   0.740000 (  0.744358)
new   0.550000   0.000000   0.550000 (  0.553690)
Speedup AS::Inflector.camelize

Benchmark:
          user     system      total        real
old   5.960000   0.020000   5.980000 (  5.981754)
new   5.740000   0.030000   5.770000 (  5.757201)
Show real LoadError on helpers require

When helper try to require missing file rails will throw exception about
missing helper.

  # app/helpers/my_helper.rb

  require 'missing'

  module MyHelper
  end

And when we try do load helper

  class ApplicationController
    helper :my
  end

Rails will throw exception. This is wrong because there is a helper
file.

  Missing helper file helpers/my_helper.rb

Now when helper try to require non-existed file rails will throw proper
exception.

  No such file to load -- missing
Speed up AS::Inflector.underscore

Benchmark:
          user     system      total        real
old   6.090000   0.120000   6.210000 (  6.202039)
new   5.930000   0.110000   6.040000 (  6.042022)
Merge pull request #10642 from LTe/invalid-load-error

Show real LoadError on helpers require
Speed up String#truncate

Benchmark:
          user     system      total        real
old   1.550000   0.040000   1.590000 (  1.585866)
new   1.250000   0.040000   1.290000 (  1.287693)
Speed up Array#split

Benchmark:
          user     system      total        real
old   0.510000   0.000000   0.510000 (  0.506749)
new   0.330000   0.000000   0.330000 (  0.336187)