#11031: Add --testdir to specify where to find tests

Patch by Sandro Tosi.  The main purpose of this option is to allow
an alternate set of tests files to be used when running tests
of the regrtest tool itself.

Lib/test/regrtest.py

@@ -42,6 +42,9 @@ Selecting tests
                 -- specify which special resource intensive tests to run
 -M/--memlimit LIMIT
                 -- run very large memory-consuming tests
+   --testdir DIR
+                -- execute test files in the specified directory (instead
+                   of the Python stdlib test suite)
 
 Special runs
 
@@ -265,7 +268,7 @@ def main(tests=None, testdir=None, verbose=0, quiet=False,
              'use=', 'threshold=', 'trace', 'coverdir=', 'nocoverdir',
              'runleaks', 'huntrleaks=', 'memlimit=', 'randseed=',
              'multiprocess=', 'coverage', 'slaveargs=', 'forever', 'debug',
-             'start=', 'nowindows', 'header'])
+             'start=', 'nowindows', 'header', 'testdir='])
     except getopt.error as msg:
         usage(msg)
 
@@ -395,6 +398,10 @@ def main(tests=None, testdir=None, verbose=0, quiet=False,
             print()   # Force a newline (just in case)
             print(json.dumps(result))
             sys.exit(0)
+        elif o == '--testdir':
+            # CWD is replaced with a temporary dir before calling main(), so we
+            # join it with the saved CWD so it ends up where the user expects.
+            testdir = os.path.join(support.SAVEDCWD, a)
         else:
             print(("No handler for option {}.  Please report this as a bug "
                    "at http://bugs.python.org.").format(o), file=sys.stderr)
@@ -469,7 +476,13 @@ def main(tests=None, testdir=None, verbose=0, quiet=False,
         print("==  ", os.getcwd())
         print("Testing with flags:", sys.flags)
 
-    alltests = findtests(testdir, stdtests, nottests)
+    # if testdir is set, then we are not running the python tests suite, so
+    # don't add default tests to be executed or skipped (pass empty values)
+    if testdir:
+        alltests = findtests(testdir, list(), set())
+    else:
+        alltests = findtests(testdir, stdtests, nottests)
+
     selected = tests or args or alltests
     if single:
         selected = selected[:1]
@@ -715,6 +728,8 @@ def main(tests=None, testdir=None, verbose=0, quiet=False,
     sys.exit(len(bad) > 0 or interrupted)
 
 
+# small set of tests to determine if we have a basically functioning interpreter
+# (i.e. if any of these fail, then anything else is likely to follow)
 STDTESTS = [
     'test_grammar',
     'test_opcodes',
@@ -727,6 +742,7 @@ STDTESTS = [
     'test_doctest2',
 ]
 
+# set of tests that we don't want to be executed when using regrtest
 NOTTESTS = {
     'test_future1',
     'test_future2',