internal/plugintest: fix CopyDir paths on Windows

[massimomazzariol]

Related Issue

Fixes #672

Description

CopyDir currently builds filesystem paths using path.Join, which always uses forward slashes.

When copyWorkingDir passes a baseDirName derived from native Windows paths, the subsequent substring check can compare paths containing different separators. This causes matching entries to be skipped without returning an error, leaving persisted step_N directories without the expected working directory contents.

This change uses filepath.Join for filesystem paths so that the generated paths use the native platform separator.

A regression test was added using a non-empty baseDirName to verify that the expected nested file is copied successfully.

Testing

The affected package and the existing persisted working directory tests pass with both Go versions used by the repository CI matrix:

• GOTOOLCHAIN=go1.25.12 go test ./internal/plugintest -count=1
• GOTOOLCHAIN=go1.25.12 go test ./helper/resource -run PersistWorkingDir -count=1
• GOTOOLCHAIN=go1.26.5 go test ./internal/plugintest -count=1
• GOTOOLCHAIN=go1.26.5 go test ./helper/resource -run PersistWorkingDir -count=1

The new regression test was also verified to fail before the production change and pass afterward.

Tested with:

• Windows 11 Pro 10.0.26200
• Terraform 1.15.8
• Go 1.25.12 and 1.26.5
• Git 2.54.0.windows.1

Other unrelated Windows-specific unit test portability failures are tracked separately in #673.

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

No changes to security controls.