Commonize demangle test pieces; allow for running specific tests and test cases; use linker sets to simplify test definition

*** 54,63 ****
  [/opt/util-tests/tests/grep_test]
  [/opt/util-tests/tests/date_test]
  [/opt/util-tests/tests/chown_test]
  
  [/opt/util-tests/tests/demangle]
! tests = ['afl-fast', 'gcc-libstdc++', 'llvm-stdcxxabi']
  
  [/opt/util-tests/tests/ctf]
  pre = precheck
  tests = [ 'ctftest' ]
--- 54,63 ----
  [/opt/util-tests/tests/grep_test]
  [/opt/util-tests/tests/date_test]
  [/opt/util-tests/tests/chown_test]
  
  [/opt/util-tests/tests/demangle]
! tests = ['c++', 'rust']
  
  [/opt/util-tests/tests/ctf]
  pre = precheck
  tests = [ 'ctftest' ]