Fix flaky test_gc_of_remote_layers (#7647)

Fixes flaky test `test_gc_of_remote_layers`, which was failing because
of the `Nothing to GC` pageserver log.
I looked into the fails, it seems that backround `gc_loop` sometimes
started GC for initial tenant, which wasn't
configured to disable GC. The fix is to not create initial tenant with
enabled gc at all.

Fixes #7538

test_runner/regress/test_layer_eviction.py

@@ -159,7 +159,9 @@ def test_basic_eviction(
 def test_gc_of_remote_layers(neon_env_builder: NeonEnvBuilder):
     neon_env_builder.enable_pageserver_remote_storage(RemoteStorageKind.LOCAL_FS)
 
-    env = neon_env_builder.init_start()
+    # don't create initial tenant, we'll create it manually with custom config
+    env = neon_env_builder.init_configs()
+    env.start()
 
     tenant_config = {
         "pitr_interval": "1s",  # set to non-zero, so GC actually does something