Only need libunwind for testing (#2131)

### Notice
This change does not modify the FIPS module boundary. This change only
avoids having CMake check for the availability of `Libunwind` when no
tests are being built.

### Description of changes:
* Libunwind is only needed when building tests.
* See same change made for main: #2093

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and the ISC license.

CMakeLists.txt

@@ -141,7 +141,7 @@ if(NOT DISABLE_PERL)
   find_package(Perl REQUIRED)
 endif()
 
-if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux" AND NOT CMAKE_CROSSCOMPILING)
+if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux" AND BUILD_TESTING AND NOT CMAKE_CROSSCOMPILING)
   find_package(PkgConfig QUIET)
   if (PkgConfig_FOUND)
     pkg_check_modules(LIBUNWIND libunwind-generic)