test-backend: Enable single-threaded profiling when thread count is unspecified.

If the number of threads is not specified
while profiling then use a single thread. This is
because profiling across multiple threads (earlier
default behaviour) may obscure the accurate
measurement of which functions are the most costly
due to thread blocking.

Signed-off-by: Akshat <akshat25iiit@gmail.com>

tools/test-backend

@@ -395,7 +395,12 @@ def main() -> None:
     # dynamic resolution of the test runner type with the django-stubs mypy plugin.
     TestRunner = cast("Type[Runner]", get_runner(settings))
 
-    parallel = default_parallel if options.processes is None else options.processes
+    if options.processes:
+        parallel = options.processes
+    elif options.profile:
+        parallel = 1
+    else:
+        parallel = default_parallel
     if parallel > 1:
         print(f"-- Running tests in parallel mode with {parallel} processes.", flush=True)
     else: