tests: Improve assertion failure messages

Consider the following test:

  a=500
  b=90
  expect_less_or_equal $a $b

This code produced the following, not very user-friendly error:

  FAILURE [2014-02-14 13:04:17] Expected: '500' <= '90'

This commit introduces more descriptive messages, ie:

  FAILURE [2014-02-14 13:04:47] Assertion 'expect_less_or_equal $a $b' failed
  Expected: '500' <= '90'
  Location: test_assert.sh:3

If the custom message is used, eg:

  MESSAGE="Checking simple arithmetics" expect_less_or_equal $a $b

the error message will look like this:

  FAILURE [2014-02-14 13:04:47] Checking simple arithmetics: Assertion 'expect_less_or_equal $a $b' failed
  Expected: '500' <= '90'
  Location: test_assert.sh:3