Tests/RunCMake: Add option for dynamic expected output

Add `RunCMake_TEST_EXPECT_*` variables for this.

Tests/RunCMake/RunCMake.cmake

@@ -47,6 +47,8 @@ function(run_cmake test)
     elseif(EXISTS ${top_src}/${test}-${o}.txt)
       file(READ ${top_src}/${test}-${o}.txt expect_${o})
       string(REGEX REPLACE "\n+$" "" expect_${o} "${expect_${o}}")
+    elseif(DEFINED RunCMake_TEST_EXPECT_${o})
+      string(REGEX REPLACE "\n+$" "" expect_${o} "${RunCMake_TEST_EXPECT_${o}}")
     else()
       unset(expect_${o})
     endif()