Add readme to tests. Add fileDir option to testament & create nimble test.

tests/readme.md

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+This directory contains the test cases.
+
+Each test must have a filename of the form: ``t*.nim``
+
+**Note:** Tests are only compiled by default. In order to get the tester to
+execute the compiled binary, you need to specify a spec with an ``action`` key
+(see below for details).
+
+# Specs
+
+Each test can contain a spec in a ``discard """ ... """`` block.
+
+**Check out the [``parseSpec`` procedure](https://github.com/nim-lang/Nim/blob/devel/tests/testament/specs.nim#L124) in the ``specs`` module for a full and reliable reference**
+
+## action
+
+Specifies what action this test should take.
+
+**Default: compile**
+
+Options:
+
+* ``compile`` - compiles the module and fails the test if compilations fails.
+* ``run`` - compiles and runs the module, fails the test if compilation or
+            execution of test code fails.
+* ``reject`` - compiles the module and fails the test if compilation succeeds.
+
+There are certain spec keys that imply ``run``, including ``output`` and
+``outputsub``.
+
+## cmd
+
+Specifies the Nim command to use for compiling the test.
+
+There are a number of variables that are replaced in this spec option:
+
+* ``$target`` - the compilation target, e.g. ``c``.
+* ``$options`` - the options for the compiler.
+* ``$file`` - the filename of the test.
+* ``$filedir`` - the directory of the test file.
+
+Example:
+
+```nim
+discard """
+  cmd: "nim $target --nimblePath:./nimbleDir/simplePkgs $options $file"
+"""
+```
+
+# Categories
+
+Each folder under this directory represents a test category, which can be
+tested by running `koch tests cat <category>`.
+
+The folder ``rodfiles`` contains special tests that test incremental
+compilation via symbol files.
+
+The folder ``dll`` contains simple DLL tests.
+
+The folder ``realtimeGC`` contains a test for validating that the realtime GC
+can run properly without linking against the nimrtl.dll/so. It includes a C
+client and platform specific build files for manual compilation.