added tests for str mode

tests/str.rs

@@ -0,0 +1,35 @@
+use dir_embed::Embed;
+
+#[derive(Embed)]
+#[dir = "./../testdata/"]
+#[mode = "str"]
+pub struct Assets;
+
+#[test]
+fn str_get() {
+    assert!(Assets::get("file1.txt").is_some());
+}
+
+#[test]
+fn str_get_missing() {
+    assert!(Assets::get("missing.txt").is_none());
+}
+
+#[test]
+fn str_read_content() {
+    let content_should = "file1";
+    let content_is = Assets::get("file1.txt").unwrap();
+    assert_eq!(content_is, content_should);
+}
+
+#[test]
+fn str_sub_directories_get() {
+    assert!(Assets::get("sub/file2.txt").is_some());
+}
+
+#[test]
+fn str_sub_directories_content() {
+    let content_should = "file2";
+    let content_is = Assets::get("sub/file2.txt").unwrap();
+    assert_eq!(content_is, content_should);
+}