tests/functional2: stop littering TMPDIR so much

Nearly each tests gets a fresh store (in the test root residing under TMPDIR),
which with default settings means 8 MiB of reserved space to be deleted by the
user in case of space shortages, which of course for a test suite is just
wasted. Stop doing that to considerably reduce the litter (experimentally, from
3.7 GiB to 67 MiB per run). The old functional test suite already did the same.

Change-Id: I912cdda6e796ea37cfee0b8fe0478e976a6a6964

tests/functional2/testlib/fixtures/nix.py

@@ -45,6 +45,11 @@ class NixSettings:
 
     def to_config(self, env: ManagedEnv) -> str:
         config = dedent(f"""
+            # Running the test suite creates a lot of stores in the test root (somewhere under TMPDIR).
+            # Obviously, they are not critical for system operation, so there is no need to reserve space.
+            # The cleanup will only happen a couple of runs later, wasting space in the meantime.
+            # Effectively disable this space reserve to reduce the waste considerably (by about 98%).
+            gc-reserved-space = 0
             show-trace = true
             sandbox = true
             # explicitly disable substitution by default, otherwise we may attempt to contact