chore: fix racy test (#13196)

The application in test 
1. imports profiler which would result in setting
`bootstrap.profiler._scheduler._last_export` to a value > 0.
2. and sets the variable to a sentinel value from the module.

This code is racy and could lead to unexpected failure, if 2. happens
after 1. which was the case in [perf(profiling): code provenance outputs
directories/.py files instead of all seen
files](https://github.com/DataDog/dd-trace-py/pull/12616/files)


https://gitlab.ddbuild.io/DataDog/apm-reliability/dd-trace-py/-/jobs/895566565


## Checklist
- [x] PR author has checked that all the criteria below are met
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- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [ ] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
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[release branch maintenance
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Author: taegyunkim

tests/contrib/gunicorn/wsgi_mw_app.py

@@ -2,6 +2,7 @@
 This app exists to replicate and report on failures and degraded behavior that can arise when using ddtrace with
 gunicorn
 """
+
 import os
 
 
@@ -19,10 +20,6 @@
 
 tracer.configure(trace_processors=[PingFilter()])
 
-SCHEDULER_SENTINEL = -1
-assert bootstrap.profiler._scheduler._last_export not in (None, SCHEDULER_SENTINEL)
-bootstrap.profiler._scheduler._last_export = SCHEDULER_SENTINEL
-
 
 def aggressive_shutdown():
     tracer.shutdown(timeout=1)
@@ -37,7 +34,12 @@ def simple_app(environ, start_response):
         data = b"goodbye"
     else:
         payload = {
-            "profiler": {"is_active": bootstrap.profiler._scheduler._last_export != SCHEDULER_SENTINEL},
+            "profiler": {
+                # Once the scheduler is initialized, the last_export is set to a
+                # timestamp using time.time_ns()
+                "is_active": bootstrap.profiler._scheduler._last_export
+                > 0,
+            },
         }
         data = json.dumps(payload).encode("utf-8")