Add a `count_uniq` sanity function

[vkarak]

I think it is useful for flexible tests since it avoids caveats like the following:

import reframe as rfm
import reframe.utility.sanity as sn


@rfm.simple_test
class HostnameCheck(rfm.RunOnlyRegressionTest):
    def __init__(self):
        super().__init__()
        self.valid_systems = ['daint', 'dom']
        self.valid_prog_environs = ['PrgEnv-cray']
        self.executable = 'hostname'
        self.sourcesdir = None
        self.num_tasks = 0
        self.sanity_patterns = sn.assert_eq(
            self.num_tasks_assigned,
            sn.count(sn.findall(r'nid\d+', self.stdout))
        )

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

The test passes fine, but it's missing the num_tasks_per_node = 1, which would have been caught easily if I could replace count with count_uniq:

nid00000
nid00000
nid00000
nid00000
nid00000
nid00000
nid00000
nid00000
nid00001
nid00001
nid00001
nid00001
nid00001
nid00001
nid00001