test/fs_test.go: use require instead of assert

[invidian]

Otherwise test suite will panic when an operation fails, which next steps depend on. This change should ensure clearer error messages and less false-negative test results.

The operation may fail e.g. when tests run is interrupted early.

[Copilot]

Pull request overview

This PR improves test reliability by converting assert calls to require for critical operations where subsequent test steps depend on success. This prevents panics and provides clearer error messages when tests are interrupted or fail.

Key changes:

• Added require import from testify
• Converted assert.Nil(t, err) to require.NoError(t, err) for critical operations
• Converted assert.Equal to require.Equal for values that subsequent code depends on
• Minor style improvements (variable declaration syntax, whitespace cleanup)