Tests: Test FixedArray completely

Except for tangential accessors such as data(), there is no more feature
of FixedArray that is untested after this large expansion of its test
cases. These tests, with the help of the new NoAllocationGuard, also
test the allocation contract that was fixated in the last commit.

Hopefully this builds confidence in future Kernel uses of FixedArray
as well as its establishment in the real-time parts of the audio
subsystem. I'm excited :^)

AK/NoAllocationGuard.h

@@ -58,7 +58,7 @@ private:
 #endif
     }
 
-    bool m_allocation_enabled_previously;
+    bool m_allocation_enabled_previously { true };
 };
 
 }

Tests/AK/TestFixedArray.cpp

@@ -4,14 +4,16 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibTest/TestCase.h>
 #include <LibTest/TestSuite.h>
 
 #include <AK/FixedArray.h>
-#include <AK/String.h>
+#include <AK/NoAllocationGuard.h>
 
 TEST_CASE(construct)
 {
-    EXPECT(FixedArray<int>().size() == 0);
+    EXPECT_EQ(FixedArray<int>().size(), 0u);
+    EXPECT_EQ(FixedArray<int>::must_create_but_fixme_should_propagate_errors(1985).size(), 1985u);
 }
 
 TEST_CASE(ints)
@@ -24,3 +26,56 @@ TEST_CASE(ints)
     EXPECT_EQ(ints[1], 1);
     EXPECT_EQ(ints[2], 2);
 }
+
+TEST_CASE(swap)
+{
+    FixedArray<int> first = FixedArray<int>::must_create_but_fixme_should_propagate_errors(4);
+    FixedArray<int> second = FixedArray<int>::must_create_but_fixme_should_propagate_errors(5);
+    first[3] = 1;
+    second[3] = 2;
+    first.swap(second);
+    EXPECT_EQ(first.size(), 5u);
+    EXPECT_EQ(second.size(), 4u);
+    EXPECT_EQ(first[3], 2);
+    EXPECT_EQ(second[3], 1);
+}
+
+TEST_CASE(move)
+{
+    FixedArray<int> moved_from_array = FixedArray<int>::must_create_but_fixme_should_propagate_errors(6);
+    FixedArray<int> moved_to_array(move(moved_from_array));
+    EXPECT_EQ(moved_to_array.size(), 6u);
+    EXPECT_EQ(moved_from_array.size(), 0u);
+}
+
+TEST_CASE(no_allocation)
+{
+    FixedArray<int> array = FixedArray<int>::must_create_but_fixme_should_propagate_errors(5);
+    EXPECT_NO_CRASH("Assigments", [&] {
+        NoAllocationGuard guard;
+        array[0] = 0;
+        array[1] = 1;
+        array[2] = 2;
+        array[4] = array[1];
+        array[3] = array[0] + array[2];
+        return Test::Crash::Failure::DidNotCrash;
+    });
+
+    EXPECT_NO_CRASH("Move", [&] {
+        FixedArray<int> moved_from_array = FixedArray<int>::must_create_but_fixme_should_propagate_errors(6);
+        NoAllocationGuard guard;
+        FixedArray<int> moved_to_array(move(moved_from_array));
+        // We need to ensure that this destructor runs before the FixedArray destructor.
+        guard.~NoAllocationGuard();
+        return Test::Crash::Failure::DidNotCrash;
+    });
+
+    EXPECT_NO_CRASH("Swap", [&] {
+        FixedArray<int> target_for_swapping;
+        {
+            NoAllocationGuard guard;
+            array.swap(target_for_swapping);
+        }
+        return Test::Crash::Failure::DidNotCrash;
+    });
+}