Fix dealing with infinity and NaN in `test/assert.rb:assert_float`

`assert_float` is always passed when expected value and/or actual value are
infinity or NaN. This behavior seems unintentional.

Before this patch:

    assert_float(Float::INFINITY, 1.0)  #=> pass
    assert_float(-Float::INFINITY, 1)   #=> pass
    assert_float(1, 1/0)                #=> pass
    assert_float(1, -1/0)               #=> pass
    assert_float(1.0, Float::NAN)       #=> pass
    assert_float(Float::NAN, 1)         #=> pass

After this patch:

    assert_float(Float::INFINITY, 1.0)  #=> fail: Expected 1.0 to be Infinity.
    assert_float(-Float::INFINITY, 1)   #=> fail: Expected 1 to be -Infinity.
    assert_float(1, 1/0)                #=> fail: Expected Infinity to be 1.
    assert_float(1, -1/0)               #=> fail: Expected -Infinity to be 1.
    assert_float(1.0, Float::NAN)       #=> fail: Expected NaN to be 1.0.
    assert_float(Float::NAN, 1)         #=> fail: Expected 1 to be NaN.

test/assert.rb

@@ -148,10 +148,15 @@ end
 ##
 # Fails unless +exp+ is equal to +act+ in terms of a Float
 def assert_float(exp, act, msg = nil)
-  unless ret = check_float(exp, act)
-    diff = "    Expected |#{exp} - #{act}| (#{(exp-act).abs}) to be <= #{Mrbtest::FLOAT_TOLERANCE}."
+  e, a = exp.to_f, act.to_f
+  if (e.infinite? || a.infinite?) && e != a ||
+     e.nan? && !a.nan? || !e.nan? && a.nan?
+    assert_true(false, msg, "    Expected #{act} to be #{exp}.")
+  elsif (n = (e - a).abs) > Mrbtest::FLOAT_TOLERANCE
+    assert_true(false, msg, "    Expected |#{exp} - #{act}| (#{n}) to be <= #{Mrbtest::FLOAT_TOLERANCE}.")
+  else
+    assert_true(true)
   end
-  assert_true(ret, msg, diff)
 end
 
 def assert_raise(*exc)
@@ -211,19 +216,6 @@ def report
   $ko_test == 0 && $kill_test == 0
 end
 
-##
-# Performs fuzzy check for equality on methods returning floats
-def check_float(a, b)
-  tolerance = Mrbtest::FLOAT_TOLERANCE
-  a = a.to_f
-  b = b.to_f
-  if a.finite? and b.finite?
-    (a-b).abs < tolerance
-  else
-    true
-  end
-end
-
 def _eval_assertion(meth, exp, act_or_msg, msg, block)
   if block
     exp, act, msg = exp, block.call, act_or_msg