Inc refactoring, Value helper test method extractions, and more integration helpers. (#1135)

* Manifests check. Ignore doctests for now.

* We continue with refactorings towards the separation of concerns between
crates. `nu_plugin_inc` and `nu_plugin_str` common test helpers usage
has been refactored into `nu-plugin` value test helpers.

Inc also uses the new API for integration tests.

crates/nu_plugin_inc/src/lib.rs

@@ -0,0 +1,38 @@
+mod inc;
+mod nu_plugin_inc;
+
+pub use inc::Inc;
+
+#[cfg(test)]
+mod tests {
+    use super::Inc;
+    use crate::inc::Action;
+    use nu_protocol::Value;
+    use nu_value_ext::ValueExt;
+
+    impl Inc {
+        pub fn expect_action(&self, action: Action) {
+            match &self.action {
+                Some(set) if set == &action => {}
+                Some(other) => panic!(format!("\nExpected {:#?}\n\ngot {:#?}", action, other)),
+                None => panic!(format!("\nAction {:#?} not found.", action)),
+            }
+        }
+
+        pub fn expect_field(&self, field: Value) {
+            let field = match field.as_column_path() {
+                Ok(column_path) => column_path,
+                Err(reason) => panic!(format!(
+                    "\nExpected {:#?} to be a ColumnPath, \n\ngot {:#?}",
+                    field, reason
+                )),
+            };
+
+            match &self.field {
+                Some(column_path) if column_path == &field => {}
+                Some(other) => panic!(format!("\nExpected {:#?} \n\ngot {:#?}", field, other)),
+                None => panic!(format!("\nField {:#?} not found.", field)),
+            }
+        }
+    }
+}