migrate export migration tests from python to go

[leehinman]

Proposed commit message

Converts test_migration.py system tests to Go integration tests using the existing libbeat/tests/integration framework, continuing the effort to replace Python-based system tests with Go integration tests.

• Adds TestMigrationDefault, TestMigrationFalse, and TestMigrationTrue in libbeat/tests/integration/cmd_export_migration_test.go
• Removes libbeat/tests/system/test_migration.py

The three tests cover the behaviour of the migration.6_to_7.enabled config flag when running export template:

Test	migration.6_to_7.enabled	migration_alias_false	migration_alias_true
TestMigrationDefault	unset (default: off)	present	absent
TestMigrationFalse	false	present	absent
TestMigrationTrue	true	present	present

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

Disruptive User Impact

None. Test Code

How to test this PR locally

cd libbeat && mage goIntegTest

Related issues

• Relates Replace Python system tests with Go tests #48850

Use cases

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