pageserver: allow for unit test stress test (#11112)

## Problem

I like using `cargo stress` to hammer on a test, but it doesn't work out
of the box because it does parallel runs by default and tests always use
the same repo dir.

## Summary of changes

Add an uuid to the test repo dir when generating it.

Cargo.lock

@@ -4303,6 +4303,7 @@ dependencies = [
  "tracing",
  "url",
  "utils",
+ "uuid",
  "wal_decoder",
  "walkdir",
  "workspace_hack",