plugin tests: more informative failure messages

When a test fails, show the name of the key that failed as well as
the value of that key.

Author: cxw42

tests/plugin/spec/editorconfig_spec.rb

@@ -18,7 +18,8 @@ def test_editorconfig(vim, file_name, expected_values)
   vim.edit(File.join(BASE_PATH, file_name))
 
   expected_values.each do |key, val|
-    expect(vim.echo("&l:#{key}")).to eq(val)
+    vimval = vim.echo("&l:#{key}")
+    expect(vimval).to eq(val), "key #{key} had value #{vimval}, but I expected #{val}"
   end
 
   vim.command 'bd!'