Add ExprTk_BUILD_TESTING flag to enable testing

Also move the tests in a tests directory

tests/CMakeLists.txt

@@ -0,0 +1,18 @@
+# **************************************************************
+# *         C++ Mathematical Expression Toolkit Library        *
+# *                                                            *
+# * Author: Arash Partow (1999-2023)                           *
+# * URL: https://www.partow.net/programming/exprtk/index.html  *
+# *                                                            *
+# * Copyright notice:                                          *
+# * Free use of the Mathematical Expression Toolkit Library is *
+# * permitted under the guidelines and in accordance with the  *
+# * most current version of the MIT License.                   *
+# * http://www.opensource.org/licenses/MIT                     *
+# *                                                            *
+# **************************************************************
+
+add_executable(exprtk_test exprtk_test.cpp)
+target_link_libraries(exprtk_test ExprTk)
+
+add_test(NAME exprtk_test COMMAND exprtk_test)