bpo-35513, unittest: TextTestRunner uses time.perf_counter() (GH-11180)

TextTestRunner of unittest.runner now uses time.perf_counter() rather
than time.time() to measure the execution time of a test: time.time()
can go backwards, whereas time.perf_counter() is monotonic.

Similar change made in libregrtest, pprint and random.
diff --git a/Lib/pprint.py b/Lib/pprint.py
index bcf2eed..f2a1178 100644
--- a/Lib/pprint.py
+++ b/Lib/pprint.py
@@ -568,11 +568,11 @@
     if object is None:
         object = [("string", (1, 2), [3, 4], {5: 6, 7: 8})] * 100000
     p = PrettyPrinter()
-    t1 = time.time()
+    t1 = time.perf_counter()
     _safe_repr(object, {}, None, 0)
-    t2 = time.time()
+    t2 = time.perf_counter()
     p.pformat(object)
-    t3 = time.time()
+    t3 = time.perf_counter()
     print("_safe_repr:", t2 - t1)
     print("pformat:", t3 - t2)
 
diff --git a/Lib/random.py b/Lib/random.py
index 03c058a..9c2904c 100644
--- a/Lib/random.py
+++ b/Lib/random.py
@@ -740,14 +740,14 @@
     sqsum = 0.0
     smallest = 1e10
     largest = -1e10
-    t0 = time.time()
+    t0 = time.perf_counter()
     for i in range(n):
         x = func(*args)
         total += x
         sqsum = sqsum + x*x
         smallest = min(x, smallest)
         largest = max(x, largest)
-    t1 = time.time()
+    t1 = time.perf_counter()
     print(round(t1-t0, 3), 'sec,', end=' ')
     avg = total/n
     stddev = _sqrt(sqsum/n - avg*avg)
diff --git a/Lib/test/libregrtest/runtest.py b/Lib/test/libregrtest/runtest.py
index 466b522..dc2abf2 100644
--- a/Lib/test/libregrtest/runtest.py
+++ b/Lib/test/libregrtest/runtest.py
@@ -162,7 +162,7 @@
         abstest = get_abs_module(ns, test)
         clear_caches()
         with saved_test_environment(test, ns.verbose, ns.quiet, pgo=ns.pgo) as environment:
-            start_time = time.time()
+            start_time = time.perf_counter()
             the_module = importlib.import_module(abstest)
             # If the test has a test_main, that will run the appropriate
             # tests.  If not, use normal unittest test loading.
@@ -180,7 +180,7 @@
                 refleak = dash_R(the_module, test, test_runner, ns.huntrleaks)
             else:
                 test_runner()
-            test_time = time.time() - start_time
+            test_time = time.perf_counter() - start_time
         post_test_cleanup()
     except support.ResourceDenied as msg:
         if not ns.quiet and not ns.pgo:
diff --git a/Lib/test/time_hashlib.py b/Lib/test/time_hashlib.py
index 2585ecb..55ebac6 100644
--- a/Lib/test/time_hashlib.py
+++ b/Lib/test/time_hashlib.py
@@ -14,26 +14,26 @@
     longStr = b'Z'*scale
 
     localCF = creatorFunc
-    start = time.time()
+    start = time.perf_counter()
     for f in range(iterations):
         x = localCF(longStr).digest()
-    end = time.time()
+    end = time.perf_counter()
 
     print(('%2.2f' % (end-start)), "seconds", iterations, "x", len(longStr), "bytes", name)
 
 def test_create():
-    start = time.time()
+    start = time.perf_counter()
     for f in range(20000):
         d = creatorFunc()
-    end = time.time()
+    end = time.perf_counter()
 
     print(('%2.2f' % (end-start)), "seconds", '[20000 creations]')
 
 def test_zero():
-    start = time.time()
+    start = time.perf_counter()
     for f in range(20000):
         x = creatorFunc().digest()
-    end = time.time()
+    end = time.perf_counter()
 
     print(('%2.2f' % (end-start)), "seconds", '[20000 "" digests]')
 
diff --git a/Lib/unittest/runner.py b/Lib/unittest/runner.py
index 2c5ea4a..45e7e4c 100644
--- a/Lib/unittest/runner.py
+++ b/Lib/unittest/runner.py
@@ -168,7 +168,7 @@
                     warnings.filterwarnings('module',
                             category=DeprecationWarning,
                             message=r'Please use assert\w+ instead.')
-            startTime = time.time()
+            startTime = time.perf_counter()
             startTestRun = getattr(result, 'startTestRun', None)
             if startTestRun is not None:
                 startTestRun()
@@ -178,7 +178,7 @@
                 stopTestRun = getattr(result, 'stopTestRun', None)
                 if stopTestRun is not None:
                     stopTestRun()
-            stopTime = time.time()
+            stopTime = time.perf_counter()
         timeTaken = stopTime - startTime
         result.printErrors()
         if hasattr(result, 'separator2'):