Attempt at fixing unit tests

This fixes some of the tests by limiting the output of the ClangFormat
helper function to between `line1` and `line2`. This is similar to what
`clang_format#format` now returns.

There are two problems left:
1. the number of lines returned by `clang_format#format` might not be
   equal to `line2 - line1`, i.e when it splits a line.
2. `clang_format#replace` now handles reidenting the lines correctly, so
   any tests that format a line that is indented will fail.

Ref: rhysd#61

t/clang_format_spec.vim

@@ -48,11 +48,12 @@ function! ClangFormat(line1, line2, ...) abort
     let opt .= "}'"
 
     let cmd = printf('%s %s ./%st/%s --', g:clang_format#command, opt, s:root_dir, file)
-    let result = Chomp(system(cmd))
+    let result = systemlist(cmd)
     if v:shell_error
         throw "Error on system(): clang-format exited with non-zero.\nCommand: " . cmd . "\nOutput: " . result
     endif
-    return Chomp(system(cmd))
+    let l = filter(result, { idx -> (idx + 1) >= a:line1 && (idx + 1) <= a:line2 })
+    return Chomp(join(l, "\n"))
 endfunction
 "}}}