Add some vader tests for completion feature

I want to improve file/path completion, so add some tests to cover
existing behaviour (at least existing behaviour as I understand it).

test/feature/completion.vader

@@ -0,0 +1,61 @@
+Before:
+  enew!
+  only!
+  cd include/completion
+  let w:testing = 1
+
+After:
+  cd -
+
+Execute (command: flags, match all):
+  Assert len(grepper#complete('-', '', v:null)) > 3
+  AssertEqual count(grepper#complete('-', '', v:null), '-cword '), 1
+  AssertEqual count(grepper#complete('-', '', v:null), '-grepprg '), 1
+  AssertEqual count(grepper#complete('-', '', v:null), '-tool '), 1
+
+Execute (command: flags, match one):
+  AssertEqual grepper#complete('-noquickf', '', v:null), ['-noquickfix ']
+
+Execute (command: flags, match some):
+  Assert len(grepper#complete('-no', '', v:null)) > 2
+  AssertEqual count(grepper#complete('-no', '', v:null), '-switch '), 0
+  AssertEqual count(grepper#complete('-no', '', v:null), '-noswitch '), 1
+
+Execute (command: flags, -dir options):
+  Assert len(grepper#complete('', 'Grepper -dir ', v:null)) > 2
+  AssertEqual count(grepper#complete('', 'Grepper -dir ', v:null), 'cwd '), 1
+  AssertEqual count(grepper#complete('', 'Grepper -dir ', v:null), 'repo '), 1
+
+Execute (command: flags, -stop completion):
+  Assert grepper#complete('', 'Grepper -stop ', v:null)[0] =~# '\d\+'
+
+Execute (command: flags, -tool options):
+  Assert len(grepper#complete('', 'Grepper -tool ', v:null)) > 1
+  AssertEqual count(grepper#complete('', 'Grepper -tool ', v:null), 'grep '), 1
+
+Execute (command: flags, -tool options):
+  Assert len(grepper#complete('', 'Grepper -tool ', v:null)) > 1
+  AssertEqual count(grepper#complete('', 'Grepper -tool ', v:null), 'grep '), 1
+
+Execute (prompt: relative path, empty string):
+  AssertEqual grepper#complete_files('x ', v:null, v:null), ['x ./foo/']
+
+Execute (prompt: relative path, whitespace only):
+  AssertEqual grepper#complete_files('x  ', v:null, v:null), ['x  ./foo/']
+
+Execute (prompt: sub-path, ./ exact match):
+  AssertEqual grepper#complete_files('x ./', v:null, v:null), ['x ./foo/']
+
+Execute (prompt: sub-path, ./ and word chars):
+  AssertEqual grepper#complete_files('x ./f', v:null, v:null), ['x ./foo/']
+
+Execute (prompt: nested sub-path, e.g. ./foo/):
+  AssertEqual grepper#complete_files('x ./foo/', v:null, v:null), ['x ./foo/bar/']
+
+Execute (prompt: absolute path, / exact match):
+  AssertEqual count(grepper#complete_files('x /', v:null, v:null), 'x /bin/'), 1
+  AssertEqual count(grepper#complete_files('x /', v:null, v:null), 'x /var/'), 1
+
+Execute (prompt: absolute path, / and word chars):
+  AssertEqual count(grepper#complete_files('x /b', v:null, v:null), 'x /bin/'), 1
+  AssertEqual count(grepper#complete_files('x /b', v:null, v:null), 'x /var/'), 0