Update integration tests to use extended grep instead of PCRE

[clue]

Builds on top of clue/framework-x#283, #42, #41 and #40

[Copilot]

Pull Request Overview

This PR updates integration tests to replace PCRE (Perl Compatible Regular Expressions) with extended grep patterns for better compatibility and portability.

• Replaces -P (PCRE) flags with -E (extended regex) in grep commands
• Adds skipifnot() function for conditional test skipping
• Updates regex patterns to work with extended grep syntax
• Modifies curl output handling to strip carriage returns

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