Run specs from within the spec directory

This means all reported paths will be relative, which makes it much
easier to move specs without having to regenerate expected error
messages.

lib/sass_spec/engine_adapter.rb

@@ -55,7 +55,12 @@ def version
 
   def compile(sass_filename, style, precision)
     cmd = "#{@command} --precision #{precision} -t #{style || "expanded"}"
-    result = capture3_with_timeout("#{cmd} #{sass_filename}", :binmode => true, :timeout => @timeout)
+
+    dirname, basename = File.split(sass_filename)
+    result = Dir.chdir(dirname) do
+      capture3_with_timeout("#{File.absolute_path(cmd)} #{basename}",
+        binmode: true, timeout: @timeout)
+    end
 
     if result[:timeout]
       ["", "Execution timed out after #{@timeout}s", -1]
@@ -79,6 +84,11 @@ def initialize(path)
 
         main() async {
           await for (var line in new LineSplitter().bind(utf8.decoder.bind(stdin))) {
+            if (line.startsWith("!cd ")) {
+              Directory.current = line.substring("!cd ".length);
+              continue;
+            }
+
             try {
               await sass.main(line.split(" ").where((arg) => arg.isNotEmpty).toList());
             } catch (error, stackTrace) {
@@ -111,7 +121,9 @@ def version
   end
 
   def compile(sass_filename, style, precision)
-    @stdin.puts "--no-color --no-unicode --style #{style || 'expanded'} #{@args} #{sass_filename}"
+    dirname, basename = File.split(sass_filename)
+    @stdin.puts "!cd #{File.absolute_path(dirname)}"
+    @stdin.puts "--no-color --no-unicode --style #{style || 'expanded'} #{@args} #{basename}"
     [next_chunk(@stdout), next_chunk(@stderr), next_chunk(@stdout).to_i]
   end
 

lib/sass_spec/util.rb

@@ -8,6 +8,7 @@ def normalize_output(css)
 
     # Normalizes the path names and whitespace in the given error message.
     def normalize_error(error)
+      # TODO(nweiz): Delete path normalization when sass/libsass#2861 is fixed.
       error.gsub(/(?:\/todo_|_todo\/)/, "/") # hide todo pre/suffix
         .gsub(/\/libsass\-[a-z]+\-tests\//, "/") # hide test directory
         .gsub(/\/libsass\-[a-z]+\-issues\//, "/libsass-issues/") # normalize issue specs