title: How to set the tolerance of expect_equal in testthat framework
body: <p>I would like to know if it is possible in R using <code>testthat</code> test framework to set the tolerance for equality.</p>
  
  <p>Currently, if <code>example.R</code> is:</p>
  
  <pre><code>library(testthat)
  three_times&lt;-function(x) 3*x
  
  context('Test three_times')
  test_that('Three times returns 3 times x',{
      expect_equal(three_times(3),9)
      expect_equal(three_times(pi),9.4247)
  })
  </code></pre>
  
  <p>and executed with <code>test_file('example.R','stop')</code>, the first test passes, but the second fails with:</p>
  
  <pre><code> Error: Test failed: 'Three times returns 3 times x'
  Not expected: three_times(pi) not equal to 9.4247
  Mean relative difference: 8.271963e-06. 
  </code></pre>
  
  <p>Is it possible to set a higher error threshold for the mean relative difference? for example  1e-3. I have some expected results with only a 3 decimal precision, meaning now my tests always fail...</p>