Update ISBN verifier

This updates the exercise with new test cases from
exercism/problem-specifications#2604 and
exercism/problem-specifications#2601.

Author: kahgoh

exercises/practice/isbn-verifier/.meta/tests.toml

@@ -30,6 +30,12 @@ description = "invalid character in isbn is not treated as zero"
 [28025280-2c39-4092-9719-f3234b89c627]
 description = "X is only valid as a check digit"
 
+[8005b57f-f194-44ee-88d2-a77ac4142591]
+description = "only one check digit is allowed"
+
+[fdb14c99-4cf8-43c5-b06d-eb1638eff343]
+description = "X is not substituted by the value 10"
+
 [f6294e61-7e79-46b3-977b-f48789a4945b]
 description = "valid isbn without separating dashes"
 

exercises/practice/isbn-verifier/src/test/java/IsbnVerifierTest.java

@@ -68,6 +68,20 @@ public void xIsOnlyValidAsACheckDigit() {
         assertThat(isbnVerifier.isValid("3-598-2X507-9")).isFalse();
     }
 
+    @Disabled("Remove to run test")
+    @Test
+    @DisplayName("only one check digit is allowed")
+    public void onlyOneCheckDigitIsAllowed() {
+        assertThat(isbnVerifier.isValid("3-598-21508-96")).isFalse();
+    }
+
+    @Disabled("Remove to run test")
+    @Test
+    @DisplayName("X is not substituted by the value 10")
+    public void xIsNotSubstitutedByTheValue10() {
+        assertThat(isbnVerifier.isValid("3-598-2X507-5")).isFalse();
+    }
+
     @Disabled("Remove to run test")
     @Test
     @DisplayName("valid isbn without separating dashes")