Rename test-unit-file to test-load-fragment

This file was testing a mix of functions from src/core/load-fragment.c and some
from src/shared/install.c. Let's name it more appropriately. I want to add
tests for the new unit-file.c too.

src/test/meson.build

@@ -148,7 +148,7 @@ tests += [
           libmount,
           libblkid]],
 
-        [['src/test/test-unit-file.c',
+        [['src/test/test-load-fragment.c',
           'src/test/test-helper.c'],
          [libcore,
           libshared],