axfer: add a section about unit test

Two unit tests are available to check internal implementation of axfer.
They perform file I/O for many times and take much time to finish.

This commit adds a section about it.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

axfer/axfer-transfer.1

@@ -971,6 +971,22 @@ some scenarios below, no copying occurs between modules.
  - xfer(mmap/interleaved), mapper(single), container(any)
  - xfer(mmap/non\-interleaved), mapper(multiple), containers(any)
 
+.SS Unit test
+
+For each of the
+.I mapper
+and
+.I container
+module, unit test is available. To run the tests, execute below command:
+
+.nf
+$ make test
+.fi
+
+Each test iterates writing to file and reading to the file for many times and it
+takes long time to finish. Please take care of the execution time if running on
+any CI environment.
+
 .SH SEE ALSO
 \fB
 axfer(1),

axfer/axfer.1

@@ -75,6 +75,13 @@ Operates for playback transmission.
 .I EXIT_FAILURE
 (1).
 
+.SH UNIT TEST
+
+This program has unit tests for internal implementation. Please refer to the
+manual of
+.I axfer-transfer
+for details.
+
 .SH COMPATIBILITY TO APLAY
 
 The