tmt tests convert traceback for stray Makefile

[lukaszachy]

When you run tmt tests convert in directory which has Makefile but it is not Beakerlib one you get traceback. For example if it is ran from tmt directory:

$ tmt tests convert
Checking the '/home/lzachar/_important_/Gits/tmt' directory.
Makefile found in '/home/lzachar/_important_/Gits/tmt/Makefile'.
Traceback (most recent call last):
  File "/home/lzachar/.virtualenvs/tmt/bin/tmt", line 7, in <module>
    exec(compile(f.read(), __file__, 'exec'))
  File "/home/lzachar/_important_/Gits/tmt/bin/tmt", line 11, in <module>
    tmt.cli.main()
  File "/home/lzachar/.virtualenvs/tmt/lib/python3.7/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/home/lzachar/.virtualenvs/tmt/lib/python3.7/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/home/lzachar/.virtualenvs/tmt/lib/python3.7/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/lzachar/.virtualenvs/tmt/lib/python3.7/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/lzachar/.virtualenvs/tmt/lib/python3.7/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/lzachar/.virtualenvs/tmt/lib/python3.7/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/lzachar/.virtualenvs/tmt/lib/python3.7/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/lzachar/_important_/Gits/tmt/tmt/cli.py", line 447, in convert
    common, individual = tmt.convert.read(path, makefile, nitrate, purpose)
  File "/home/lzachar/_important_/Gits/tmt/tmt/convert.py", line 77, in read
    beaker_task = re.search('export TEST=(.*)\n', content).group(1)
AttributeError: 'NoneType' object has no attribute 'group'