parallel-tests: separate different logs with an empty line

parallel-tests: separate different logs with an empty line

After commit v1.12-21-g5eeb366 "parallel-tests: optimize global log
creation", the log files copied in the global log are not anymore
separated by a blank line, causing unclear output like this:

  FAIL: test1
  ===========

  output from test 1
  FAIL: test2
  ===========

  output from test 2

where we would want something like this:

  FAIL: test1
  ===========

  output from test 1

  FAIL: test2
  ===========

  output from test 2

Fix this regression.

* lib/am/check.am (am__create_global_log): Print an extra empty line
after having displayed the content of a '.log' file.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>