Added Unit Tests

Added unit tests to verify that when the `brk` syscall is called with illegal arguments (causing a stack-heap collision), the syscall returns a non-zero value, indicating that it was not executed successfully.

test/riscv-tests-64/brk_syscall.S

@@ -0,0 +1,35 @@
+.text
+ .globl _start
+ _start: nop
+
+#-------------------------------------------------------------
+# Stack-Heap Collision brk syscall Test
+#-------------------------------------------------------------
+
+test_0: # Test Ecall Return Value
+    mv a0, sp # Assign the ecall parameter to the stack pointer value 
+  	lw a7, BRK # Load the brk syscall immediate value
+    ecall
+
+    # If the return value of the ecall is zero, then the ecall
+    # was executed successfully and therefore the test failed
+    beqz a0, fail
+
+  bne x0, gp, pass
+
+pass: 
+ li a0, 42
+ li a7, 93
+ ecall
+
+
+fail: 
+ li a0, 0
+ li a7, 93
+ ecall
+
+.data
+.align 4
+
+# Syscall immediate value
+BRK: .word 214

test/riscv-tests/brk_syscall.s

@@ -0,0 +1,27 @@
+.text
+
+#-------------------------------------------------------------
+# Stack-Heap Collision brk syscall Test
+#-------------------------------------------------------------
+
+test_0: # Test Ecall Return Value
+    mv a0, sp # Assign the ecall parameter to the stack pointer value 
+  	li a7, 214 # Load the brk syscall immediate value
+    ecall
+
+    # If the return value of the ecall is zero, then the ecall
+    # was executed successfully and therefore the test failed
+    beqz a0, fail
+
+  bne x0, gp, pass
+
+pass: 
+ li a0, 42
+ li a7, 93
+ ecall
+
+
+fail: 
+ li a0, 0
+ li a7, 93
+ ecall