Make test.cpp support LOGFAIL env var: only failed tests show log.

Set LOGFAIL= one of ALL, DEBUG, INFO, WARN, ERROR, NONE. A passing test will
run silently, as now; but a failing test will replay log output at the
specified level or higher.
While at it, support LOGTEST environment variable, same values. This is like
setting --debug (or -d), but allows specifying an arbitrary level -- and,
unlike --debug, can be set for a TeamCity build config without modifying any
scripts or code.
Publish LLError::decodeLevel(std::string), previously private to llerror.cpp.