Add support for skipped robot tests

[SAmorozov]

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Context

Now all robot's tests with tag "skipped" are marked as skipped in allure report.

Checklist

• Sign Allure CLA
• Provide unit tests

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[sseliverstov]

Thank you for your PR!

As you know, robot hasn't any possibilities to mark test for skipping, except if you directly select tests to execute in cmd line. In your PR status just redefined to skipped, so I not sure if it right. Test skipping means that it isn't actually runned.

For example, if the test doing something dangerous like table truncating then your reason to skip it is prevent dangerous action.

Is it true, you don't want to see some fails in report? Why are you not use --exclude?

[SAmorozov]

Hi!
Yes, robot can't mark test as skipped, but we can do it in test by adding tag "skipped" and catching it in reporting servises, such as allure. It needed when we have functionality, that can be disabled. So our test previously checks, if functionality is enabled or disabled, and continue checking or skipping itself.
When we have many tests and many stands, we can't check manually if functionality is enabled or disabled, that's why we can't use --exclude.

[skhomuti]

@sseliverstov Example of skip_execution function

    def skip_execution(self, message, *tags):
        """Skip execution with pass result

        Args:
            message: message explaining why execution skipped
            tags: modifyed tags; see `pass_execution` keyword in `BuiltIn` library of Robot Framework
        """
        self.builtin_lib.log("Skip tests:%s" % message, level='WARN')
        try:
            self.builtin_lib.pass_execution("Skip tests:%s" % message, "skipped", *tags)
        except RuntimeError:
            self.pass_execution_without_tags(message)