CPack test: expand output checking fallback

Some tests can have the same output for a certain
subtest no matter which packaging generator is used.

Tests/RunCMake/CPack/CPackTestHelpers.cmake

@@ -68,6 +68,8 @@ function(run_cpack_test_common_ TEST_NAME types build SUBTEST_SUFFIX source PACK
         set(RunCMake-std${o}-file "tests/${TEST_NAME}/${TEST_TYPE}-${SUBTEST_SUFFIX}-std${o}.txt")
       elseif(EXISTS ${RunCMake_SOURCE_DIR}/tests/${TEST_NAME}/${TEST_TYPE}-std${o}.txt)
         set(RunCMake-std${o}-file "tests/${TEST_NAME}/${TEST_TYPE}-std${o}.txt")
+      elseif(SUBTEST_SUFFIX AND EXISTS ${RunCMake_SOURCE_DIR}/tests/${TEST_NAME}/${SUBTEST_SUFFIX}-std${o}.txt)
+        set(RunCMake-std${o}-file "tests/${TEST_NAME}/${SUBTEST_SUFFIX}-std${o}.txt")
       elseif(EXISTS ${RunCMake_SOURCE_DIR}/${TEST_TYPE}/default_expected_std${o}.txt)
         set(RunCMake-std${o}-file "${TEST_TYPE}/default_expected_std${o}.txt")
       endif()

Tests/RunCMake/CPack/README.txt

@@ -167,6 +167,7 @@ NOTE: For subtests generator name can also be suffixed with subtest name and/or
         - generator name + packaging type
         - generator name + subtest name
         - generator name
+        - subtest name
         - default generator
       File name format: '<generator_name>-<packaging_type>-<subtest_name>-std<type>.txt'
                         where <type> can either be 'out' or 'err'.