fix tests broken on Windows due to path separator

Author: rhysd

testdata/projects/local_action_invalid.out

@@ -1,4 +1,4 @@
 /workflows/test\.yaml:7:15: name is required in action metadata "testdata(\\\\|/)projects(\\\\|/)local_action_invalid(\\\\|/)no_name(\\\\|/)action\.yaml" \[action\]/
 /workflows/test\.yaml:8:15: description is required in metadata of "My action" action at "testdata(\\\\|/)projects(\\\\|/)local_action_invalid(\\\\|/)no_desc(\\\\|/)action\.yaml" \[action\]/
-/workflows/test\.yaml:9:15: incorrect icon name "does-not-exist" at branding\.icon in metadata of "Incorrect branding" action at "testdata/projects/local_action_invalid/branding/action\.yaml"\. see the official document to know the exhaustive list of supported icons: https://.+ \[action\]/
-/workflows/test\.yaml:9:15: incorrect color "no-color" at branding\.icon in metadata of "Incorrect branding" action at "testdata/projects/local_action_invalid/branding/action\.yaml"\. see the official document to know the exhaustive list of supported colors: https://.+ \[action\]/
+/workflows/test\.yaml:9:15: incorrect icon name "does-not-exist" at branding\.icon in metadata of "Incorrect branding" action at "testdata(\\\\|/)projects(\\\\|/)local_action_invalid(\\\\|/)branding(\\\\|/)action\.yaml"\. see the official document to know the exhaustive list of supported icons: https://.+ \[action\]/
+/workflows/test\.yaml:9:15: incorrect color "no-color" at branding\.icon in metadata of "Incorrect branding" action at "testdata(\\\\|/)projects(\\\\|/)local_action_invalid(\\\\|/)branding(\\\\|/)action\.yaml"\. see the official document to know the exhaustive list of supported colors: https://.+ \[action\]/