ctest: add option for output truncation

Add `--test-output-truncation` to `ctest`. This option can be used to
customize which part of the test output is being truncated. Currently
supported values are `tail`, `middle` and `head`.

Also add equivalent `CTEST_CUSTOM_TEST_OUTPUT_TRUNCATION` variable.

Fixes: #23206

Help/variable/CTEST_CUSTOM_MAXIMUM_FAILED_TEST_OUTPUT_SIZE.rst

@@ -3,7 +3,8 @@ CTEST_CUSTOM_MAXIMUM_FAILED_TEST_OUTPUT_SIZE
 
 When saving a failing test's output, this is the maximum size, in bytes, that
 will be collected by the :command:`ctest_test` command. Defaults to 307200
-(300 KiB).
+(300 KiB). See :variable:`CTEST_CUSTOM_TEST_OUTPUT_TRUNCATION` for possible
+truncation modes.
 
 If a test's output contains the literal string "CTEST_FULL_OUTPUT",
 the output will not be truncated and may exceed the maximum size.

Help/variable/CTEST_CUSTOM_MAXIMUM_PASSED_TEST_OUTPUT_SIZE.rst

@@ -3,7 +3,8 @@ CTEST_CUSTOM_MAXIMUM_PASSED_TEST_OUTPUT_SIZE
 
 When saving a passing test's output, this is the maximum size, in bytes, that
 will be collected by the :command:`ctest_test` command. Defaults to 1024
-(1 KiB).
+(1 KiB). See :variable:`CTEST_CUSTOM_TEST_OUTPUT_TRUNCATION` for possible
+truncation modes.
 
 If a test's output contains the literal string "CTEST_FULL_OUTPUT",
 the output will not be truncated and may exceed the maximum size.

Help/variable/CTEST_CUSTOM_TEST_OUTPUT_TRUNCATION.rst

@@ -0,0 +1,12 @@
+CTEST_CUSTOM_TEST_OUTPUT_TRUNCATION
+-----------------------------------
+
+.. versionadded:: 3.24
+
+Set the test output truncation mode in case a maximum size is configured
+via the :variable:`CTEST_CUSTOM_MAXIMUM_PASSED_TEST_OUTPUT_SIZE` or
+:variable:`CTEST_CUSTOM_MAXIMUM_FAILED_TEST_OUTPUT_SIZE` variables.
+By default the ``tail`` of the output will be truncated. Other possible
+values are ``middle`` and ``head``.
+
+.. include:: CTEST_CUSTOM_XXX.txt