add section describing success and failure unittest output

Author: glennj

docs/TESTS.md

@@ -21,4 +21,60 @@ Working through each exercise is a process of:
 *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.
 
-@TODO add IDE related instructions.
+## 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
+```
+
+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. -->