Allow profiling tests with tracing instrumentation (#2957)
To get more insights into test performance, allow instrumenting tests
with tracing-durations-export.
Usage:
```shell
# A single test
TRACING_DURATIONS_TEST_ROOT=$(pwd)/target/test-traces cargo test --features tracing-durations-export --test pip_install_scenarios no_binary -- --exact
# All tests
TRACING_DURATIONS_TEST_ROOT=$(pwd)/target/test-traces cargo nextest run --features tracing-durations-export
```
Then we can e.g. look at
`target/test-traces/pip_install_scenarios::no_binary.svg` and see the
builds it performs:
