Add tests for chapter1-3.

tests/chapter1/hello_world.rs

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+/// Run this as a application on RV64 bare-metal.
+
+fn main() {
+    println!("Hello, world!");
+}

tests/chapter2/power.rs

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+/// rCore application without user_lib.
+/// Load it manually to 0x80040000.
+/// It should OK.
+
+const SIZE: usize = 10;
+const P: u32 = 3;
+const STEP: usize = 10000000;
+const MOD: u32 = 10007;
+fn main() -> ! {
+    let mut pow = [0u32; SIZE];
+    let mut index: usize = 0;
+    pow[index] = 1;
+    for i in 1..=STEP {
+        let last = pow[index];
+        index = (index + 1) % SIZE;
+        pow[index] = last * P % MOD;
+        if i % 10000 == 0 {
+            println!("{}^{}={}", P, i, pow[index]);
+        }
+    }
+    println!("Test power OK!");
+    loop {}
+}

tests/chapter2/power_bad.rs

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+/// rCore application without user_lib.
+/// Load it manually to 0x80040000.
+/// It should not OK. Kernel should catch the page fault.
+
+const SIZE: usize = 10;
+const P: u32 = 3;
+const STEP: usize = 10000000;
+const MOD: u32 = 10007;
+fn main() -> ! {
+    let mut pow = [0u32; SIZE];
+    let mut index: usize = 0;
+    pow[index] = 1;
+    for i in 1..=STEP {
+        let last = pow[index];
+        index = (index + 1) % SIZE;
+        pow[index] = last * P % MOD;
+        if i % 10000 == 0 {
+            println!("{}^{}={}", P, i, pow[index]);
+        }
+        if i == STEP / 2 {
+            // TODO: Modify satp to a malicious value in order to destroy memory access mechanism.
+        }
+    }
+    println!("Test power_bad OK!");
+    loop {}
+}

tests/chapter3/write_a.rs

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+/// rCore application without user_lib.
+/// Load it manually to 0x80040000.
+
+use user_lib::{sys_yield};
+
+fn main() -> ! {
+    for _ in 0..10 {
+        for _ in 0..10 {
+            print!("A");
+        }
+        println!();
+        sys_yield();
+    }
+    loop {}
+}

tests/chapter3/write_b.rs

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+/// rCore application without user_lib.
+/// Load it manually to 0x80050000.
+
+use user_lib::{sys_yield};
+
+fn main() -> ! {
+    for _ in 0..10 {
+        for _ in 0..10 {
+            print!("B");
+        }
+        println!();
+        sys_yield();
+    }
+    loop {}
+}