Add support for full regexp --grep arguments

[tux-rampage]

Motivation

Some IDEs (i.e. WebStorm) pass a full regexp, including the delimitters, to karma run (See WI-55539). This will cause the runner to skip all tests instead of applying the pattern.

This PR aims to provide compatibility for this behaviour.

Example

karma run --grep=/^some\\.test\\.Suite/

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[tux-rampage]

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