tests: only stat a test file if we don't already know its "cost"

tests/run-tests.py

@@ -1845,18 +1845,24 b' class TestRunner(object):'
                     b'gendoc': 100,
                     b'contrib-perf': 200,
                    }
+            perf = {}
             def sortkey(f):
                 # run largest tests first, as they tend to take the longest
                 try:
+                    return perf[f]
+                except KeyError:
+                    try:
                     val = -os.stat(f).st_size
                 except OSError as e:
                     if e.errno != errno.ENOENT:
                         raise
-                    return -1e9 # file does not exist, tell early
+                        perf[f] = -1e9 # file does not exist, tell early
+                        return -1e9
                 for kw, mul in slow.items():
                     if kw in f:
                         val *= mul
-                return val
+                    perf[f] = val
+                    return perf[f]
             tests.sort(key=sortkey)
 
         self._testdir = osenvironb[b'TESTDIR'] = getattr(