test: pin the number of CPU for issue4074 tests

On machine with an hundreds of CPUs, the "user" CPU time reported can be
inflated by the status steps. Since the test especially focus on the diff
computation, we restrict the number of CPU to avoid potential issues.

Differential Revision: https://phab.mercurial-scm.org/D8112

tests/test-issue4074.t

@@ -25,5 +25,5 b' Modify it:'
 
 Time a check-in, should never take more than 10 seconds user time:
 
-  $ hg ci --time -m1
+  $ hg ci --time -m1 --config worker.enabled=no
   time: real .* secs .user [0-9][.].* sys .* (re)