run-tests: run tests with as many processes as cores by default

This seems like a useful default behavior so tests run faster by default*

* Except in special circumstances where the OS/filesystem doesn't scale well
  to many CPU cores (like APFS *cough* *cough*).

Differential Revision: https://phab.mercurial-scm.org/D5071

tests/run-tests.py

@@ -51,6 +51,7 b' import difflib'
 import distutils.version as version
 import errno
 import json
+import multiprocessing
 import os
 import random
 import re
@@ -287,7 +288,7 b" if 'java' in sys.platform:"
     IMPL_PATH = b'JYTHONPATH'
 
 defaults = {
-    'jobs': ('HGTEST_JOBS', 1),
+    'jobs': ('HGTEST_JOBS', multiprocessing.cpu_count()),
     'timeout': ('HGTEST_TIMEOUT', 180),
     'slowtimeout': ('HGTEST_SLOWTIMEOUT', 500),
     'port': ('HGTEST_PORT', 20059),

tests/test-run-tests.t

@@ -15,7 +15,7 b' Define a helper to avoid the install ste'
 =============
   $ rt()
   > {
-  >     "$PYTHON" $TESTDIR/run-tests.py --with-hg=`which hg` "$@"
+  >     "$PYTHON" $TESTDIR/run-tests.py --with-hg=`which hg` -j1 "$@"
   > }
 
 error paths
@@ -1329,7 +1329,7 b' Test globbing of local IP addresses'
 Add support for external test formatter
 =======================================
 
-  $ CUSTOM_TEST_RESULT=basic_test_result "$PYTHON" $TESTDIR/run-tests.py --with-hg=`which hg` "$@" test-success.t test-failure.t
+  $ CUSTOM_TEST_RESULT=basic_test_result "$PYTHON" $TESTDIR/run-tests.py --with-hg=`which hg` -j1 "$@" test-success.t test-failure.t
   running 2 tests using 1 parallel processes 
   
   # Ran 2 tests, 0 skipped, 0 failed.