Make virtualenvwrapper test directories collision-proof

[karthiknadig]

Problem

The macOS ARM PR job intermittently fails pet-virtualenvwrapper::environments::tests::get_project_reads_existing_project_path_from_project_file because get_project() observes a missing project/prefix path during parallel tests.

create_test_dir() builds names from process ID plus wall-clock nanoseconds. Multiple tests use the same logical name (wrapped-env), and clock resolution can collide under parallel execution, allowing one test to remove another test's directory.

Observed in PR #495 job 92445735042.

Acceptance criteria

• Add a process-local atomic sequence to test directory names.
• Preserve existing cleanup behavior.
• Run the pet-virtualenvwrapper tests repeatedly/concurrently.