tests: add test-path

It tests all available directives of Path units:
- PathChanged
- PathModified
- PathExists
- PathExisysGlob
- DirectoryNotEmpty
- MakeDirectory
- DirectoryMode
- Unit
build-sys: avoid duplication of macro definition

TEST_DIR is already defined in AM_CFLAGS
core: remove unused macro GC_QUEUE_USEC_MAX

It is unused since cf1265e188e876dda906dca0029248a06dc80c33
build-sys: link to libsystemd-core only when needed

Multiple executables do not need libsystemd-core
update .gitignore
build-sys: do not include tests in code coverage