tests: don't match blank output lines as regexes in unified tests

Currently, the following unified test will pass:

  $ echo foo


A blank output line (a line containing just two spaces) will match any
output.

The patch modifies the unified test runner to ignore empty strings
strings when do regular expression matching.

tests/run-tests.py

@@ -530,7 +530,7 b' def tsttest(test, options):'
 
             if el == l: # perfect match (fast)
                 postout.append("  " + l)
-            elif el and rematch(el, l): # fallback regex match
+            elif el and el[2:] and rematch(el, l): # fallback regex match
                 postout.append("  " + el)
             else: # mismatch - let diff deal with it
                 postout.append("  " + l)