[feat] Add new syntax for attaching arbitrary regression test methods to pipeline stages

[vkarak]

This PR offers a new syntax for attaching arbitrary regression test methods to pipeline stages. This is achieved using the @run_before(stage) or @run_after(stage) decorators, where stage is a string representing a pipeline stage. Valid pipeline stages are the following: setup, compile, run, sanity, performance and cleanup. This syntax will render less common or even obsolete the overriding of the pipeline methods, such as setup() for doing common tasks, such as setting the compilation flags. Users will not have to remember the signatures of the pipeline methods and to always call super(). For example, setting the compilation flags based on the programming environment can now be written very nicely as follows:

@rfm.simple_test
class Example2bTest(rfm.RegressionTest):
    def __init__(self):
        self.descr = 'Matrix-vector multiplication example with OpenMP'
        self.valid_systems = ['*']
        self.valid_prog_environs = ['PrgEnv-cray', 'PrgEnv-gnu',
                                    'PrgEnv-intel', 'PrgEnv-pgi']
        self.sourcepath = 'example_matrix_vector_multiplication_openmp.c'
        self.build_system = 'SingleSource'
        self.executable_opts = ['1024', '100']
        self.prgenv_flags = {
            'PrgEnv-cray':  ['-homp'],
            'PrgEnv-gnu':   ['-fopenmp'],
            'PrgEnv-intel': ['-openmp'],
            'PrgEnv-pgi':   ['-mp']
        }
        self.variables = {
            'OMP_NUM_THREADS': '4'
        }
        self.sanity_patterns = sn.assert_found(
            r'time for single matrix vector multiplication', self.stdout)
        self.maintainers = ['you-can-type-your-email-here']
        self.tags = {'tutorial'}

    @rfm.run_before('compile')
    def setflags(self):
        self.build_system.cflags = self.prgenv_flags[self.current_environ.name]

Some pipeline stages, namely compile and run, are implemented as two phases: (a) launch the operation asynchronously and (b) wait for the operation to complete. In such cases the @run_after decorated functions will run after the stage is completed (i.e., after it is waited for).

This is still WIP, because I need to update the documentation. Other than that, you are free to review.

[victorusu]

lgtm