Make aggregate_tests callable from project dir

This is Python testing convention and allows coverage to correctly
record the paths of the source files and left-strip the parts
leading to the project directory.

This commit also replaces manual test loading with the unittest
`discover` function, which adds some path magic, so that tests
are actually run over the source code in the project dir.

For the sake of simplified test discovery, this commit removes
randomization of test module loading.
It also does not seem beneficial to just shuffle the test
modules and not the test cases or test methods.

tests/.coveragerc → .coveragerc

@@ -12,3 +12,5 @@ exclude_lines =
 
 omit =
   */securesystemslib/_vendor/*
+  */tests/*
+  */site-packages/*

.travis.yml

@@ -20,7 +20,4 @@ script:
   - tox
 
 after_success:
-  - cd tests
   - coveralls
-  - cd -
-

tests/aggregate_tests.py

@@ -20,7 +20,7 @@
 
 <Purpose>
   Run all the unit tests from every .py file beginning with "test_" in
-  'tuf/tests'.  Use --random to run the tests in random order.
+  'securesystemslib/tests'.
 """
 
 # Help with Python 3 compatibility, where the print statement is a function, an
@@ -33,29 +33,9 @@
 
 import sys
 import unittest
-import glob
-import random
-
-# Generate a list of pathnames that match a pattern (i.e., that begin with
-# 'test_' and end with '.py'.  A shell-style wildcard is used with glob() to
-# match desired filenames.  All the tests matching the pattern will be loaded
-# and run in a test suite.
-tests_list = glob.glob('test_*.py')
-
-# Remove '.py' from each filename to allow loadTestsFromNames() (called below)
-# to properly load the file as a module.
-tests_without_extension = []
-for test in tests_list:
-  test = test[:-3]
-  tests_without_extension.append(test)
-
-# Randomize the order in which the tests run.  Randomization might catch errors
-# with unit tests that do not properly clean up or restore monkey-patched
-# modules.
-random.shuffle(tests_without_extension)
 
 if __name__ == '__main__':
-  suite = unittest.TestLoader().loadTestsFromNames(tests_without_extension)
+  suite = unittest.TestLoader().discover("tests", top_level_dir=".")
   all_tests_passed = unittest.TextTestRunner(verbosity=1).run(suite).wasSuccessful()
   if not all_tests_passed:
     sys.exit(1)

tox.ini

@@ -7,10 +7,9 @@
 envlist = py27, py34, py35, py36
 
 [testenv]
-changedir = tests
 
 commands =
-    coverage run --source securesystemslib aggregate_tests.py
+    coverage run tests/aggregate_tests.py
     coverage report -m --fail-under 100
 
 deps =