test-path: drop PATH_TEST() macro

test-path: drop PATH_TEST() macro

The macro was introduced by fc7a32df38dd4797ce7266df6ece6bd9b9842783,
but unfortunately gcc/gcov/lcov are confused by the macro:
```
lcov: ERROR: lcov: ERROR: (inconsistent) mismatched end line for test_path_exists_body at
/home/runner/work/systemd/systemd/src/test/test-path.c:158:
158 -> 185 while capturing from build/mkosi.builddir/arch~rolling~x86-64/test-path.p/src_test_test-path.c.gcno
(use "lcov --ignore-errors inconsistent ..." to bypass this error)
```

The macro deos nothing spurious, hence it should be a bug in gcc.
But, the macro just reduces the lines for calling setup_test(), and
does not simplify test cases so much.
Let's call it in each test cases, and stop using the macro.