Add flag to skip tests for non-unix systems

cmd/tfplugindocs/testdata/scripts/generate/framework_provider_success_generic_templates.txtar

@@ -1,5 +1,6 @@
 # Successful run of tfplugindocs on a Framework provider with "generic" template paths (i.e. templates/resources.md.tmpl)
 # Templates test all implemented data fields and functions.
+[!unix] skip
 env GOCACHE=$WORK/gocache
 env GOMODCACHE=$WORK/gomodcache
 exec tfplugindocs --provider-name=terraform-provider-scaffolding

cmd/tfplugindocs/testdata/scripts/generate/framework_provider_success_named_templates.txtar

@@ -1,5 +1,6 @@
 # Successful run of tfplugindocs on a Framework provider with "named" template paths (i.e. templates/resources/<resource name>.md.tmpl)
 # Templates test all implemented data fields and functions.
+[!unix] skip
 env GOCACHE=$WORK/gocache
 env GOMODCACHE=$WORK/gomodcache
 exec tfplugindocs --provider-name=terraform-provider-scaffolding

cmd/tfplugindocs/testdata/scripts/generate/framework_provider_success_no_templates.txtar

@@ -1,4 +1,5 @@
 # Successful run of tfplugindocs on a Framework provider with examples but no templates or pre-exiting docs.
+[!unix] skip
 env GOCACHE=$WORK/gocache
 env GOMODCACHE=$WORK/gomodcache
 exec tfplugindocs --provider-name=terraform-provider-scaffolding

cmd/tfplugindocs/testdata/scripts/generate/null_provider_success.txtar

@@ -1,5 +1,6 @@
 # Successful run of tfplugindocs on a stripped-down version of the Null provider with pre-exiting templates, examples,
 # docs, and non-tfplugindocs managed docs (docs/cdktf subdirectory).
+[!unix] skip
 env GOCACHE=$WORK/gocache
 env GOMODCACHE=$WORK/gomodcache
 exec tfplugindocs --provider-name=terraform-provider-null