feat: Add run() wrapper with command output assertions

[aarvio]

Add a new test pattern for asserting command outputs and exit status through a run() wrapper function and corresponding assertion functions.

Addresses issue: #134

User story suggested pipe syntax:
command_under_test | should_display "${expected_pattern}"
command_under_test | err_should_contain "${expected_pattern}"

Implemented wrapper pattern:
run command_under_test && assert_run_output_matches "${expected_pattern}"
run command_under_test && assert_run_error_matches "${expected_pattern}"

Works also as separate lines:
run command_under_test
assert_run_output_matches "${expected_pattern}"
assert_run_error_matches "${expected_pattern}"

The wrapper pattern achieves the same readability while providing more flexibility and reliability.

Problem with pipes:

• pipes carry only stdout
• harder to resolve exit status if needed
• can't verify multiple conditions independently

Usage Examples

Simple one-liners:
run my_script && assert_run_success
run my_command && assert_run_output_equals "expected"
run failing_cmd && assert_run_status_code 42

Multiple assertions:
run my_script.sh input.txt
assert_run_success
assert_run_output_matches "^Processed.*successfully"
assert_run_error_equals ""