Add tags to force-style tests

Adds an optional "tags" entry in the force style test YAML. This is a
comma-separated list of keywords, which are parsed by CMake and added as labels
for CTest.  This allows more fine-grained filtering of tests. Any
newly-generated YAML file automatically adds the "generated" tag.

unittest/force-styles/test_main.cpp

@@ -46,6 +46,9 @@ void write_yaml_header(YamlWriter *writer, TestConfig *cfg, const char *version)
     // lammps_version
     writer->emit("lammps_version", version);
 
+    // tags
+    writer->emit("tags", cfg->tags_line());
+
     // date_generated
     std::time_t now   = time(nullptr);
     std::string block = trim(ctime(&now));