Make `make_deferrable` available as a sanity function

[vkarak]

This is quite a useful function that converts any expression or variable to a deferrable. A usage scenario is the following:

import reframe.utility.sanity as sn
from reframe.core.deferrable import make_deferrable

class SomeFlexibleTest(...):
    def __init__():
        ...
        self.sanity_patterns = sn.assert_eq(sn.count(...), make_deferrable(self.num_tasks))

num_tasks in such a case will be set at the run phase, so you need to defer its evaluation when writing the sanity patterns. Currently, you have to either import this function separately as above, or write your sanity function, e.g.,:

import reframe.utility.sanity as sn

class SomeFlexibleTest(...):
    def __init__():
        ...
        self.sanity_patterns = sn.assert_eq(sn.count(...), self.num_tasks_assigned)

    @property 
    @sn.sanity_function
    def num_tasks_assigned(self):
        return self.num_tasks