tests: add support for listing tests to run in a file

This will make it easier for us to run the tests in the python3
whitelist in buildbot.

tests/run-tests.py

@@ -303,6 +303,8 b' def getparser():'
         help="skip tests listed in the specified blacklist file")
     parser.add_option("--whitelist", action="append",
         help="always run tests listed in the specified whitelist file")
+    parser.add_option("--test-list", action="append",
+                      help="read tests to run from the specified file")
     parser.add_option("--changed", type="string",
         help="run tests that are changed in parent rev or working directory")
     parser.add_option("-C", "--annotate", action="store_true",
@@ -2279,6 +2281,10 b' class TestRunner(object):'
             # positional arguments are paths to test files to run, so
             # we make sure they're all bytestrings
             args = [_bytespath(a) for a in args]
+            if options.test_list is not None:
+                for listfile in options.test_list:
+                    with open(listfile, 'rb') as f:
+                        args.extend(t for t in f.read().splitlines() if t)
             self.options = options
 
             self._checktools()

tests/test-run-tests.t

@@ -981,6 +981,19 b' Whitelist trumps blacklist'
   python hash seed: * (glob)
   [1]
 
+Ensure that --test-list causes only the tests listed in that file to
+be executed.
+  $ echo test-success.t >> onlytest
+  $ rt --test-list=onlytest
+  .
+  # Ran 1 tests, 0 skipped, 0 failed.
+  $ echo test-bogus.t >> anothertest
+  $ rt --test-list=onlytest --test-list=anothertest
+  s.
+  Skipped test-bogus.t: Doesn't exist
+  # Ran 1 tests, 1 skipped, 0 failed.
+  $ rm onlytest anothertest
+
 test for --json
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