Restore exposing machine file parsing

This was accidentally removed in
e989c83b44. I restored it and also did a
few other cleanups:

- Make a static method for namespacing purposes

- Put the test files in the data dir with the other test data

- Avoid mutating globals in the machine config tests

This will be used by Hydra.

tests/unit/libutil-support/tests/characterization.hh

@@ -12,7 +12,7 @@ namespace nix {
  * The path to the unit test data directory. See the contributing guide
  * in the manual for further details.
  */
-static Path getUnitTestData() {
+static inline Path getUnitTestData() {
     return getEnv("_NIX_TEST_UNIT_DATA").value();
 }
 
@@ -21,7 +21,7 @@ static Path getUnitTestData() {
  * against them. See the contributing guide in the manual for further
  * details.
  */
-static bool testAccept() {
+static inline bool testAccept() {
     return getEnv("_NIX_TEST_ACCEPT") == "1";
 }