textproc/source-highlight: fix "make test" or -DWITH_TESTING build

The normal build works for me, but "make test",
or poudriere-devel builds on FreeBSD 14.0-RELEASE amd64 under
WITH_TESTING=yes, fail for me in the test phase.

Fix two compiler bugs that LLVM/clang asserts:

• lib/tests/stdboosterror.h poses a most vexing parse, resolve by switching to C++11 uniform initialization (GCC 12 or 13 disambiguates by resolving to variable init.)
• lib/tests/test_wordtokenizer_main.cpp sees LLVM/clang complain about unresolved operator<< (needs to be defined earlier, so move the declaration up) (couldn't test with GCC)

Serialize tests/Makefile:

• tests/Makefile* causes "No such file or directory" errors when running with MAKE_JOBS, so patch a .NOTPARALLEL into the Makefile.

Approved by: portmgr@ (blanket just-fix-it approval for failing build)