[bugfix] Remove the validation of the name of the test's base classes

[rsarm]

Closes #451.

[codecov-io]

Codecov Report

Merging #480 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #480      +/-   ##
==========================================
- Coverage   91.37%   91.36%   -0.01%     
==========================================
  Files          70       70              
  Lines        8796     8788       -8     
==========================================
- Hits         8037     8029       -8     
  Misses        759      759

Impacted Files	Coverage Δ
reframe/frontend/loader.py	93.87% <100%> (-0.47%)	⬇️

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[vkarak]

1. We will need unit tests to avoid the bug reappearing.
2. If you parse a file with a relative import, the validation crashes:

from . import foo

./reframe.py: unexpected error: 'NoneType' object has no attribute 'startswith'
Traceback (most recent call last):
  File "/users/karakasv/Devel/reframe/reframe/frontend/cli.py", line 359, in main
    checks_found = loader.load_all()
  File "/users/karakasv/Devel/reframe/reframe/frontend/loader.py", line 175, in load_all
    checks.extend(self.load_from_file(d))
  File "/users/karakasv/Devel/reframe/reframe/frontend/loader.py", line 141, in load_from_file
    if not self._validate_source(filename):
  File "/users/karakasv/Devel/reframe/reframe/frontend/loader.py", line 74, in _validate_source
    validator.visit(source_tree)
  File "/apps/common/UES/jenkins/SLES12/easybuild/software/Python-bare/3.6.5/lib/python3.6/ast.py", line 253, in visit
    return visitor(node)
  File "/apps/common/UES/jenkins/SLES12/easybuild/software/Python-bare/3.6.5/lib/python3.6/ast.py", line 261, in generic_visit
    self.visit(item)
  File "/apps/common/UES/jenkins/SLES12/easybuild/software/Python-bare/3.6.5/lib/python3.6/ast.py", line 253, in visit
    return visitor(node)
  File "/users/karakasv/Devel/reframe/reframe/frontend/loader.py", line 32, in visit_ImportFrom
    if node.module.startswith('reframe'):
AttributeError: 'NoneType' object has no attribute 'startswith'

[vkarak]

I would simply test as follows:

if node.module is not None and node.module.startswith('reframe'):
    self._has_import = True