[roottest] Use root target file in dotLibraries test command

The `root` executable should not be used directly in the test command,
because if there was no `root` CMake target, it might use the literal
string "root" in the command, potentially resolving to a ROOT install on
the system and not the ROOT from the build directory.

It is safer to use a `TARGET_FILE` generator expression here.

Author: guitargeek

roottest/root/rint/CMakeLists.txt

@@ -21,7 +21,7 @@ ROOTTEST_ADD_TEST(missingSymbol
 if (NOT MSVC)
 # ROOT-5843
 ROOTTEST_ADD_TEST(dotLibraries
-                  COMMAND root -b -q -l -e ".libraries"
+                  COMMAND $<TARGET_FILE:root> -b -q -l -e ".libraries"
                   PASSREGEX "libRint")
 endif()