Add unit tests for is_valid method

tests/tests.rs

+extern crate dmi;
+
+#[test]
+fn table_checksum_is_valid() {
+    let s = dmi::Smbios {
+        anchor: *b"_SM_",
+        checksum: 0xc2,
+        length: 0x1f,
+        major_version: 0x02,
+        minor_version: 0x07,
+        max_structure_size: 0xb8,
+        revision: 0x00,
+        formatted: [0x00, 0x00, 0x00, 0x00, 0x00],
+        inter_anchor: *b"_DMI_",
+        inter_checksum: 0x3e,
+        table_length: 0x0c15,
+        table_address: 0x000e92f0,
+        structure_count: 0x0052,
+        bcd_revision: 27,
+    };
+
+    assert!(s.is_valid());
+}
+
+#[test]
+fn table_checksum_is_invalid() {
+    let s = dmi::Smbios {
+        anchor: *b"_SM_",
+        checksum: 0x00,
+        length: 0x1f,
+        major_version: 0x02,
+        minor_version: 0x07,
+        max_structure_size: 0xb8,
+        revision: 0x00,
+        formatted: [0x00, 0x00, 0x00, 0x00, 0x00],
+        inter_anchor: *b"_DMI_",
+        inter_checksum: 0x3e,
+        table_length: 0x0c15,
+        table_address: 0x000e92f0,
+        structure_count: 0x0052,
+        bcd_revision: 27,
+    };
+
+    assert!(!s.is_valid());
+}