tests: Run custom.py from test creation helper when available

This allows creating tests both for image and non-image devices using
the same script.

tests/README.md

@@ -3,20 +3,23 @@
 `umockdev` tests use fingerprint devices mocked by [`umockdev`
 toolchain][umockdev].
 
-This document describes how to create a 'capture' test: a test that
-captures a picture of a fingerprint from the device (mocked by
-`umockdev`) and compares it with the standard one.
+This document describes how to create test cases (for USB devices). Many of
+these tests are tests for image devices, where a single image is captured
+and stored.
 
 Other kinds of `umockdev` tests can be created in a similar manner. For
 match-on-chip devices you would instead create a test specific `custom.py`
 script, capture it and store the capture to `custom.pcapng`.
 
-'Capture' Test Creation
------------------------
-A new 'capture' test is created by means of `capture.py` script:
+'capture' and 'custom' Test Creation
+------------------------------------
+
+For image devices the `capture.py` script will be used to capture one reference
+image. If the driver is a non-image driver, then a `custom.py` script should be
+created in advance, which will be run instead.
 
 1. Make sure that libfprint is built with support for the device driver
-   that you want to capture a test case for.
+   that you want to create a test case for.
 
 2. From the build directory, run tests/create-driver-test.py as root. Note
    that if you're capturing data for a driver which already has a test case
@@ -33,8 +36,9 @@ $ sudo tests/create-driver-test.py driver [variant]
 
 5. Check whether `meson test` passes with this new test.
 
-**Note.** To avoid submitting a real fingerprint, the side of finger,
-arm, or anything else producing an image with the device can be used.
+**Note.** To avoid submitting a real fingerprint when creating a 'capture' test,
+the side of finger, arm, or anything else producing an image with the device
+can be used.
 
 
 Possible Issues