Support assert error message with regex

[fernandosci]

This is a follow-up of #200

given the template

{{- tpl "{{ fail 'error message' }}" . }}

and test

suite: test tpl

templates:
  - test.yaml
tests:
  - it: tpl error message
    asserts:
      - failedTemplate:
          errorMessage: error message

it produces:

### Chart [ helm-chart ] helm-chart

 FAIL  test tpl helm-chart/tests/fail_test.yaml
        - tpl error message

                - asserts[0] `failedTemplate` fail
                        Template:       helm-chart/templates/test.yaml
                        DocumentIndex:  0
                        Expected to equal:
                                test
                        Actual:
                                error calling tpl: error during tpl function execution for "{{ fail 'error message' }}": parse error at (helm-chart/templates/test.yaml:1): 'error message'

in order to validate this message it would require defining the complete call stack including lines.

would it be possible to introduce an "errorMessageMatchRegex" for the failedTemplate assertion?

Something like this:

suite: test tpl

templates:
  - test.yaml
tests:
  - it: tpl error message
    asserts:
      - failedTemplate:
          errorMessageMatchRegex: error\smessage$

[fernandosci]

closing as duplicate of #218 and #221