Add a test/testdata/artificial directory.

test/c/std/flate.c

diff --git a/test/c/std/flate.c b/test/c/std/flate.c
index 8e4f2aa..4ed1598 100644
--- a/test/c/std/flate.c
+++ b/test/c/std/flate.c
@@ -59,10 +59,10 @@
 };
 
 golden_test flate_256_bytes_gt = {
-    .want_filename = "../../testdata/256.bytes",    //
-    .src_filename = "../../testdata/256.bytes.gz",  //
-    .src_offset0 = 20,                              //
-    .src_offset1 = 281,                             //
+    .want_filename = "../../testdata/artificial/256.bytes",    //
+    .src_filename = "../../testdata/artificial/256.bytes.gz",  //
+    .src_offset0 = 20,                                         //
+    .src_offset1 = 281,                                        //
 };
 
 golden_test flate_midsummer_gt = {

test/testdata/README.md

diff --git a/test/testdata/README.md b/test/testdata/README.md
index 75c93b8..7793b02 100644
--- a/test/testdata/README.md
+++ b/test/testdata/README.md
@@ -11,7 +11,16 @@
 
 
 
-256.bytes just holds "\x00\x01\x02\x03...\xff".
+The artificial directory holds artificially generated test data, often to
+explicitly test corner cases of various file formats. The files there usually
+come in families whose names have a common prefix, such as three files
+"foo.bar", "foo.bar.qux", "foo.bar.commentary.txt" all prefixed by "foo.bar".
+The file whose name is that prefix is usually the canonical (typically
+hand-crafted) file, and the other files are then derived from that. Outside of
+the artificial directory, the other files in this directory are typically real
+world examples of various file formats, or deriviations of them.
+
+
 
 bricks-* are various encodings of an original photo by Nigel Tao
 <nigeltao@golang.org>.

test/testdata/artificial/256.bytes

diff --git a/test/testdata/256.bytes b/test/testdata/artificial/256.bytes
similarity index 100%
rename from test/testdata/256.bytes
rename to test/testdata/artificial/256.bytes
Binary files differ

test/testdata/artificial/256.bytes.gz

diff --git a/test/testdata/256.bytes.gz b/test/testdata/artificial/256.bytes.gz
similarity index 100%
rename from test/testdata/256.bytes.gz
rename to test/testdata/artificial/256.bytes.gz
Binary files differ