tests: fix basic.OutDirectoryIsExecutable

Commit 4b4cb9c (Merge pull request #426 from perillo/fix-argv-parsing)
added a regression test, but the test failed due to ptrace errors on
Linux.
The "Operation not permitted" error was caused by kcov trying to trace
the "/bin/usr/python".

The solution is to execute an executable having user permission.
Restore the short-test.py executable, and use it instead of python.
short-test.py always exits with exit status 0.

tests/python/short-test.py

@@ -1,9 +1,9 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 
 import sys
 
 def doSomething():
-    print 'Hello world'
+    print('Hello world')
 
 
 if __name__ == '__main__':

tests/tools/basic.py

@@ -31,6 +31,10 @@ def runTest(self):
 class OutDirectoryIsExecutable(testbase.KcovTestCase):
     def runTest(self):
         self.setUp()
-        rv,o = self.do(testbase.kcov + " echo python -V")
+        # Running a system executable on Linux may cause ptrace to fails with
+        # "Operation not permitted", even with ptrace_scope set to 0.
+        # See https://www.kernel.org/doc/Documentation/security/Yama.txt
+        executable = testbase.sources + "/tests/python/short-test.py"
+        rv,o = self.do(testbase.kcov + " echo " + executable)
 
         assert rv == 0