passtest: introduce simplest possible test that passes

The idea is twofold:

1) Show that a test passes simply by not failing, and doing nothing
   means it can't fail
2) Where appropriate, use this in our own functional tests to reduce
   the selftest runtime.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
selftests: replace sleeptest with passtest

This patch replaces most of the calls to sleeptest in our functional
test with passtest, in an attempt to reduce, without invalidating the
purporse, the time it takes to run them.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Merge pull request #267 from clebergnu/example_passtest

Introduce example test: passtest
Introduce new runner option --wrapper.

Introduce a new command line option (--wrapper) to wrap
every new process created with help of module `avocado.utils.process`
inside a support script.

Wrappers can be global, so it will wrap every process or
you can set a wrapper to a specific program.

* Set wrapper to ltrace.sh on each process:
    avocado run --wrapper ltrace.sh sometest
* Set wrapper to ltrace.sh and binary myprog:
    avocado run --wrapper ltrace.sh:myprog sometest

You can provide multiples --wrapper options,
but only one global wrapper can be active at time.

For API, the class WrapProcess accepts an optional argument `wrapper`.
If it is defined, then use this parameter as the wrapper script.
If undefined, then it will look for the value inside
`runtime.CURRENT_WRAPPER` and use it.

If `runtime.CURRENT_WRAPPER` is None, then just behave like the
`SubProcess` class.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
examples.wrappers: Scripts to use with option --wrapper.

Scripts to use with command line option --wrapper.

* dummy.sh    Do nothing (use as a template).
* strace.sh   Run process on strace, output: strace.log.<pid>
* ltrace.sh   Run process on  ltrace, output: ltrace.log.<pid>
* perf.sh     Record process on perf, output: perf.data.<pid>
* valgrind.sh Run process on valgrind's memcheck, output: valgrind.log.<pid>

Usage:

  avocado run datadir --wrapper examples/wrappers/strace.sh
  avocado run datadir \
              --wrapper examples/wrappers/ltrace.sh:make \
              --wrapper examples/wrappers/perf.sh:datadir \

Signed-off-by: Rudá Moura <rmoura@redhat.com>

Valgrind.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
selftests: Add functional tests for --wrapper feature.

Add some functional tests to explore the --wrapper feature.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
docs+man: Add documentation for the --wrapper feature.

Add documentation manual page part for the command line
feature `--wrapper`.

Signed-off-by: Rudá Moura <rmoura@redhat.com>