Refactor failure test cases not to include anonymous function numbers

[kana]

Test cases to check failure messages contains stack traces like b:foo..b:bar..b:baz, line 1. Anonymous functions are written as magic numbers like 42..777..23, line 1. And these magic numbers are not consistent. Whenever the internal of vim-vspec is changed, these magic numbers are also changed randomly.

It's a boring task to modify the test cases for the changes of the magic numbers. So that it's better to exclude anonymous function numbers from test cases.

Possible approaches:

• Ignore anonymous function numbers while comparing actual results and expected results.
• Use only named functions to get consistent results.

[kana]

• It's not feasible to use only named functions.
• In most cases, anonymous function numbers appeared in v:throwpoint are functions corresponding to examples. Other numbers are usually corresponding to user-defined functions.
• The first few entries in stack traces are corresponding to internals of vim-vspec. They are not interesting to users.

So that it would be better to suppress the first few entries from stack traces instead of excluding anonymous function numbers.

For example,

• Before:

  not ok 1 - vspec correctly shows a multiline error message
  # function <SNR>1_main..vspec#test..<SNR>2_run_suites..7, line 1
  # ...

• After:

  not ok 1 - vspec correctly shows a multiline error message
  # {example}, line 1
  # ...