Add missing test cases for plainText

Tests/MarkdownTests/Base/PlainTextConvertibleMarkupTests.swift

@@ -72,4 +72,14 @@ final class PlainTextConvertibleMarkupTests: XCTestCase {
         let text = Text("OK")
         XCTAssertEqual("OK", text.plainText)
     }
+
+    func testStrikethrough() {
+        let strikethrough = Strikethrough(Text("Strikethrough"))
+        XCTAssertEqual("~Strikethrough~", strikethrough.plainText)
+    }
+
+    func testSymbolLink() {
+        let symbolLink = SymbolLink(destination: "SymbolLink")
+        XCTAssertEqual("``SymbolLink``", symbolLink.plainText)
+    }
 }

Tests/MarkdownTests/Base/RawMarkupToMarkupTests.swift

@@ -111,4 +111,19 @@ final class RawMarkupToMarkupTests: XCTestCase {
         XCTAssertNoThrow(try Link(.link(destination: "", parsedRange: nil, [])))
         XCTAssertThrowsError(try Link(.softBreak(parsedRange: nil)))
     }
+
+    func testInlineAttributes() {
+        XCTAssertNoThrow(try InlineAttributes(.inlineAttributes(attributes: "", parsedRange: nil, [])))
+        XCTAssertThrowsError(try InlineAttributes(.softBreak(parsedRange: nil)))
+    }
+
+    func testStrikethrough() {
+        XCTAssertNoThrow(try Strikethrough(.strikethrough(parsedRange: nil, [])))
+        XCTAssertThrowsError(try Strikethrough(.softBreak(parsedRange: nil)))
+    }
+
+    func testSymbolLink() {
+        XCTAssertNoThrow(try SymbolLink(.symbolLink(parsedRange: nil, destination: nil)))
+        XCTAssertThrowsError(try SymbolLink(.lineBreak(parsedRange: nil)))
+    }
 }