Add secion to Tests document describing unittest output (#289)

* Reformat Test markdown accorting to Exercism specs.
https://exercism.org/docs/building/markdown/markdown#h-layout

* add section describing success and failure unittest output

docs/TESTS.md

@@ -1,15 +1,13 @@
 # Tests
 
-
-Each exercise supplies the unit tests. You provide the implementation.
-Each file will produce a console executable that runs the tests. Running the test executable
-prints messages for each failing test and reports a non-zero exit status when tests fail.
-
+Each exercise supplies the unit tests.
+You provide the implementation.
+Each file will produce a console executable that runs the tests.
+Running the test executable prints messages for each failing test and reports a non-zero exit status when tests fail.
 
 *Note* Your code is being tested against the test suite every time you build your project.
 If your code does not pass the one or more tests but is valid D code, it will still be compiled.
 
-
 Working through each exercise is a process of:
 
 * Running the tests with `dub test`
@@ -20,9 +18,63 @@ Working through each exercise is a process of:
   * Refactor your implementation to enhance readability, reduce duplication, etc.
   * Enable the next test
 
-*Note:* D has support for unit testing direct in the language so usually the unit tests
-reside in the same file as the implementation. The unittests are run before the body of
-main and are enabled in the final executable by using the **-unittest** compiler switch.
+*Note:* D has support for unit testing direct in the language so usually the unit tests reside in the same file as the implementation.
+The unittests are run before the body of main and are enabled in the final executable by using the **-unittest** compiler switch.
+
+## Interpreting unit test output
+
+If all the tests succeed, the test output might look like this
+
+```none
+$ dub test
+             Generating test runner configuration 'dnd-character-test-library' for 'library' (library).
+    Starting Performing "unittest" build using dmd for x86_64.
+    Building dnd-character ~master: building configuration [dnd-character-test-library]
+     Linking dnd-character-test-library
+     Running dnd-character-test-library
+1 modules passed unittests
+```
+
+The test output can be difficult to read when there are test failures.
+
+```none
+$ dub test
+             Generating test runner configuration 'dnd-character-test-library' for 'library' (library).
+    Starting Performing "unittest" build using dmd for x86_64.
+    Building dnd-character ~master: building configuration [dnd-character-test-library]
+     Linking dnd-character-test-library
+     Running dnd-character-test-library
+core.exception.AssertError@source/dnd_character.d(102): unittest failure
+----------------
+??:? _d_unittestp [0x5568ff91bf05]
+source/dnd_character.d:102 void dnd_character.__unittest_L30_C1() [0x5568ff911fee]
+??:? void dnd_character.__modtest() [0x5568ff913760]
+??:? int core.runtime.runModuleUnitTests().__foreachbody6(object.ModuleInfo*) [0x5568ff928f82]
+??:? int object.ModuleInfo.opApply(scope int delegate(object.ModuleInfo*)).__lambda2(immutable(object.ModuleInfo*)) [0x5568ff9148f7]
+??:? int rt.minfo.moduleinfos_apply(scope int delegate(immutable(object.ModuleInfo*))).__foreachbody2(ref rt.sections_elf_shared.DSO) [0x5568ff92003f]
+??:? int rt.sections_elf_shared.DSO.opApply(scope int delegate(ref rt.sections_elf_shared.DSO)) [0x5568ff9200d1]
+??:? int rt.minfo.moduleinfos_apply(scope int delegate(immutable(object.ModuleInfo*))) [0x5568ff91ffcd]
+??:? int object.ModuleInfo.opApply(scope int delegate(object.ModuleInfo*)) [0x5568ff9148c9]
+??:? runModuleUnitTests [0x5568ff928db7]
+??:? void rt.dmain2._d_run_main2(char[][], ulong, extern (C) int function(char[][])*).runAll() [0x5568ff91d248]
+??:? void rt.dmain2._d_run_main2(char[][], ulong, extern (C) int function(char[][])*).tryExec(scope void delegate()) [0x5568ff91d1d5]
+??:? _d_run_main2 [0x5568ff91d13e]
+??:? _d_run_main [0x5568ff91cf27]
+/usr/include/dmd/druntime/import/core/internal/entrypoint.d:29 main [0x5568ff90f8f1]
+??:? [0x7f473154a249]
+??:? __libc_start_main [0x7f473154a304]
+??:? _start [0x5568ff90f7e0]
+1/1 modules FAILED unittests
+Error Program exited with code 1
+```
+
+There's a lot of detailed stacktrace there, but the important line is the one above `----------------`
+
+```none
+core.exception.AssertError@source/dnd_character.d(102): unittest failure
+```
 
-@TODO add IDE related instructions.
+That line tells you that the failing assertion is on **line 102** of file **dnd_character.d**.
+You can use that to pinpoint the test that is not meeting expectations.
 
+<!-- @TODO add IDE related instructions. -->