Bundle tests with applications

- For each application, add ./tests directory and __init__.py file
  within it.

- Modify test controllers (coverage.py, runtests.py) to find the new
  test directories for testing and coverage analysis.

- Move existing application-specific test modules (test_pagekite.py) to
  the newly created directories.

plinth/tests/coverage/coverage.py

@@ -37,7 +37,10 @@ import time
 SOURCE_DIRS = ['plinth'] + glob.glob('plinth/modules/*')
 
 # Files to exclude from coverage analysis and reporting
-FILES_TO_OMIT = ['plinth/tests/*.py']
+FILES_TO_OMIT = [
+    'plinth/tests/*.py',
+    'plinth/modules/*/tests/*.py'
+]
 
 # Location of coverage HTML report files
 COVERAGE_REPORT_DIR = 'plinth/tests/coverage/report'

plinth/tests/runtests.py

@@ -37,9 +37,14 @@ def run_tests(pattern=None, return_to_caller=False):
     test_runner = TestRunner()
 
     if pattern is None:
-        pattern = 'plinth.tests'
+        pattern_list = [
+            'plinth/tests',
+            'plinth/modules'
+        ]
+    else:
+        pattern_list = [pattern]
 
-    failures = test_runner.run_tests([pattern])
+    failures = test_runner.run_tests(pattern_list)
     if failures > 0 or not return_to_caller:
         sys.exit(bool(failures))