Add some comments about the crazy variants of filesystem-disk-test.

c++/src/kj/filesystem-disk-generic-test.c++

@@ -19,8 +19,18 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#if __linux__
+
 // This test compiles filesystem-disk-unix.c++ with various features #undefed, causing it to take
 // different code paths, then runs filesystem-disk-test.c++ against that.
+//
+// This test is only intended to run on Linux, but is intended to make the code behave like it
+// would on a generic flavor of Unix.
+//
+// At present this test only runs under Ekam builds. Integrating it into other builds would be
+// awkward since it #includes filesystem-disk-unix.c++, so it cannot link against that file, but
+// needs to link against the rest of KJ. Ekam "just figures it out", but other build systems would
+// require a lot of work here.
 
 #include "filesystem.h"
 #include "debug.h"
@@ -55,3 +65,5 @@
 
 #include "filesystem-disk-unix.c++"
 #include "filesystem-disk-test.c++"
+
+#endif  // __linux__

c++/src/kj/filesystem-disk-old-kernel-test.c++

@@ -29,6 +29,11 @@
 //
 // This test must be compiled as a separate program, since it alters the calling process by
 // enabling seccomp to disable the kernel features.
+//
+// At present this test only runs under Ekam builds. It *could* reasonably easily be added to the
+// autotools or cmake builds, but would require compiling a separate test binary, which is a bit
+// weird, and may lead to spurious error reports on systems that don't support seccomp for whatever
+// reason.
 
 #include <syscall.h>
 #include <unistd.h>