fix: [tests] Check custom fail msg is already set

Check if custom fail message is already set before converting expected &
actual message to strings. This will enable setting custom fail message
separately.

Add test framework description to README.

Author: manid2

README.md

@@ -11,7 +11,7 @@ retained but no new code is pushed due to me using C++ full time so I gain
 expertise in that.
 
 For these practice programs I have borrowed working code from various online
-sources and relevant credits are given in code comments. So I hope anyone who
+sources and relevant credits are given in code comments.  So I hope anyone who
 finds this repository may use it for their own learning of data structures and
 algorithms. After learning from working solutions in this repository you may
 practice solving problems on coding contest platforms with time limit.
@@ -29,7 +29,7 @@ Coding guidelines
 * Every C++ source file must build & run as a single file target the focus is
   on solving the problem correctly.
 * Each C++ compiled binary must output test result as a single line in this
-  format `Executed M implementations with N tests.`. This makes it easy to
+  format `Executed M implementations with N tests.`.  This makes it easy to
   get tests summary.
 * For debugging each binary must print test name, test case number with input
   and output in this format and only when enabled with environment variable
@@ -70,3 +70,33 @@ make cpp/arrays/2sum-01-format # format .cpp source file
 
 SHOW_TEST_OUTPUT=1 make cpp/arrays/2sum-01-check # run test with debug output
 ```
+
+Test framework
+--------------
+
+This repository uses a single header file test "framework" to write test cases
+to verify the correctness of the algorithms and various implementations.  It
+is designed to execute test cases by providing definition to `TEST()` macros
+with unique and name description to that test.  This step registers the test
+with the tests registry.
+
+To check the test conditions with expected and actual values it provides
+`CHECK_EQ()` macro.  It checks a given condition and increments test case
+count on success and reports error and exits on failure.  To convert the
+expected & actual data into printable strings helper functions such as
+`to_string()` have to be provided.  Use `SUCCESS()` & `FAIL()` macros to
+create custom checks not provided by the framework.
+
+The test output can be printed with `SHOW_OUTPUT()` macro to verify the inputs
+passed to an algorithm and the output received from it.  The output can be
+manipulated by providing the list of IO manipulators to `SET_IO()` and reset
+with `RESET_IO()`. Custom test can be provided using `SET_CUSTOM_FAIL_MSG()`
+or by providing `to_string()` overloads.
+
+The test code is initialized with `INIT_TEST_MAIN()` macro that provides
+`main()` function definition which includes `RUN_ALL_TESTS()` macro function
+to run all the registered tests.
+
+This framework removes the need to write test output and test logic code
+across the implementations significantly reducing the number of source lines
+in a single file and  improves readability.

cpp/includes/tests.h

@@ -107,8 +107,8 @@ static vector<_test *> _tests;
 	if (e op a) {                                                        \
 		SUCCESS();                                                   \
 	} else {                                                             \
-		string em = to_string(e), am = to_string(a);                 \
-		SET_CUSTOM_FAIL_MSG(em, am);                                 \
+		if (empty(this->em) && empty(this->am))                      \
+			this->em = to_string(e), this->am = to_string(a);    \
 		FAIL();                                                      \
 	}