Fibonacci.rb

# fibonacci.rb

class Euler
  def fib(n)
    curr = 0
    succ = 1
    n.downto(0) do |x|
      curr, succ = curr + succ, curr
    end
    return curr
  end
  
  def sum_fib(limit)
    sum = 0
    n = 0
    fib_n = fib(n)
    while fib_n < limit
      sum += fib_n if fib_n % 2 == 0
      n += 1
      fib_n = fib(n)
    end
    sum
  end
end

# fibonacci_test.rb

require "test/unit"
require "fibonacci"

class TestFibonacci < Test::Unit::TestCase
  def test_fib
    e = Euler.new
    assert_equal(8, e.fib(5))
    assert_equal(89, e.fib(10)) 
    assert_equal(165580141, e.fib(40))   
  end
  
  def test_sum_fib
    e = Euler.new
    assert_equal(10, e.sum_fib(10))
    assert_equal(4613732, e.sum_fib(4000000))
  end
end

The way you have it look good.

Another way would be to be to move the assignment in the while test hence making it DRYer

while fib_n = fib(n) < limit
  sum += fib_n if fib_n % 2 == 0
  n += 1
end