Workaround headless test failure on Windows OS

For headless test task, 'prism.order' property is used to choose the
graph renderer to use. Currently, we specify this property to be 'sw'.

However, this property triggers a bug of openjdk-jfx with headless
mode [1]. This property will cause Java Runtime Error for Windows OS
including AppVeyor:

    # A fatal error has been detected by the Java Runtime Environment:
    #
    #  EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x00007ffd95b64879, pid=1476, tid=2640
    #
    # JRE version: OpenJDK Runtime Environment (11.0.1+13) (build 11.0.1+13)
    # Java VM: OpenJDK 64-Bit Server VM (11.0.1+13, mixed mode, tiered, compressed oops, g1 gc, windows-amd64)
    # Problematic frame:
    # C  [javafx_font.dll+0x4879]

This bug has been identified and will be fixed in future release [2].
There is a workaround suggested which adds static initialization
blocks to load required library before any FxToolkit code [3].

Java will initialize base classes first before classes of instance
members [4] [5]. For all GUI tests, they uses GuiUnitTest or
AddressBookSystemTest as their base class. Let's add the static
initialization blocks to these two classes to solve the problem.

[1] javafxports/openjdk-jfx#66
[2] javafxports/openjdk-jfx#66 (comment)
[3] javafxports/openjdk-jfx#66 (comment)
[4] https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.4
[5] https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.5

src/test/java/seedu/address/ui/GuiUnitTest.java

@@ -13,6 +13,15 @@
  * A GUI unit test class for AddressBook.
  */
 public abstract class GuiUnitTest {
+    // TODO: Remove this workaround after using JavaFX version 13 or above
+    // This is a workaround to solve headless test failure on Windows OS
+    // Refer to https://github.com/javafxports/openjdk-jfx/issues/66 for more details.
+    static {
+        if (System.getProperty("os.name").toLowerCase().startsWith("win")) {
+            System.loadLibrary("WindowsCodecs");
+        }
+    }
+
     @Rule
     public final UiPartRule uiPartRule = new UiPartRule();
 

src/test/java/systemtests/AddressBookSystemTest.java

@@ -45,6 +45,15 @@
  * for test verification.
  */
 public abstract class AddressBookSystemTest {
+    // TODO: Remove this workaround after using JavaFX version 13 or above
+    // This is a workaround to solve headless test failure on Windows OS
+    // Refer to https://github.com/javafxports/openjdk-jfx/issues/66 for more details.
+    static {
+        if (System.getProperty("os.name").toLowerCase().startsWith("win")) {
+            System.loadLibrary("WindowsCodecs");
+        }
+    }
+
     @ClassRule
     public static ClockRule clockRule = new ClockRule();