vtc: Move disabled tests in the tests/ directory

This should make them stand out more, and help revisit them more
frequently.

bin/varnishtest/tests.disabled/README

-Tests in this directory are not executed automatically on 'make check'.
-
-The main reason for a test to live here is when the underlying problem
-isn't fixed yet, but a test case exists. This avoids breaking the rest of
-the 'make check' tests while a fix is being produced.

bin/varnishtest/tests/README

@@ -37,3 +37,9 @@ Naming scheme
 	id ~ ^x		--> VEXT tests
 
 	Coverage for individual VMODs is in "${top_srcdir}vmod/tests".
+
+Tests suffixed with .disabled are not executed automatically on 'make check'.
+
+The main reason for a test to live here is when the underlying problem
+isn't fixed yet, but a test case exists. This avoids breaking the rest of
+the 'make check' tests while a fix is being produced.