[14_10] Added test cases for src/Graphics/Types/equations.cpp

## What
Added test cases for Graphics-Types-equations.cpp
## How to test your changes?
```
xmake build equations_test
xmake r equations_test
```

tests/L3/Graphics/Types/equations_test.cpp

@@ -0,0 +1,88 @@
+/*****************************************************************************
+ * MODULE     : equations_test.cpp
+ * DESCRIPTION: Tests on equations
+ * COPYRIGHT  : (C) 2024 Zhenjun Guo
+ *******************************************************************************
+ * This software falls under the GNU general public license version 3 or later.
+ * It comes WITHOUT ANY WARRANTY WHATSOEVER. For details, see the file LICENSE
+ * in the root directory or <http://www.gnu.org/licenses/gpl-3.0.html>.
+ ******************************************************************************/
+
+#include "equations.hpp"
+#include "tm_ostream.hpp"
+#include <QtTest/QtTest>
+
+class TestEquations : public QObject {
+  Q_OBJECT
+
+private slots:
+  void test_tridiag_solve ();
+  void test_quasitridiag_solve ();
+  void test_xtridiag_solve ();
+};
+
+/*
+         A       *  x   =   y
+  ┌ 1  8        ┐ ┌ 1 ┐   ┌ 9  ┐
+  | 5  2  9     |*| 1 | = | 16 |
+  |    6  3  10 | | 1 |   | 19 |
+  └       7  3  ┘ └ 1 ┘   └ 11 ┘
+*/
+void
+TestEquations::test_tridiag_solve () {
+  const int     n= 4;
+  array<double> a= {0, 5, 6, 7}, b= {1, 2, 3, 4}, c= {8, 9, 10, 0};
+  array<point>  x (n), y (n), expected_x (n);
+
+  int i= 0;
+  for (const int& value : {9, 16, 19, 11})
+    y[i++]= as_point (value);
+
+  for (i= 0; i < n; ++i)
+    expected_x[i]= as_point (1);
+
+  tridiag_solve (a, b, c, x, y, n);
+  for (i= 0; i < n; ++i)
+    QVERIFY (x[i] == expected_x[i]);
+}
+
+void
+TestEquations::test_quasitridiag_solve () {
+  const int     n= 4;
+  array<double> a= {0, 5, 6, 7}, b= {1, 2, 3, 4}, c= {8, 9, 10, 0},
+                u= {1, 2, 3, 4}, v= {5, 6, 7, 8};
+  array<point> x (n), y (n), expected_x (n);
+
+  int i= 0;
+  for (const int& value : {35, 68, 97, 115})
+    y[i++]= as_point (value);
+
+  for (i= 0; i < n; ++i)
+    expected_x[i]= as_point (1);
+
+  quasitridiag_solve (a, b, c, u, v, x, y, n);
+  for (i= 0; i < n; ++i)
+    QVERIFY (x[i] == expected_x[i]);
+}
+
+void
+TestEquations::test_xtridiag_solve () {
+  const int     n= 4;
+  array<double> a= {0, 5, 6, 7}, b= {1, 2, 3, 4}, c= {8, 9, 10, 0};
+  double        a0= 0, a1= 1;
+  array<point>  x (n), y (n), expected_x (n);
+
+  int i= 0;
+  for (const int& value : {10, 16, 19, 11})
+    y[i++]= as_point (value);
+
+  for (i= 0; i < n; ++i)
+    expected_x[i]= as_point (1);
+
+  xtridiag_solve (a, b, c, a0, a1, x, y, n);
+  for (i= 0; i < n; ++i)
+    QVERIFY (x[i] == expected_x[i]);
+}
+
+QTEST_MAIN (TestEquations)
+#include "equations_test.moc"