test: Prevent disk space warning during node_init_tests

mzumsande pointed out bitcoin#32345 (comment) that this test was causing a warning:

   Warning: Disk space for "/tmp/test_common bitcoin/node_init_tests/init_test/bf78678cb7723a3e84b5/blocks" may not accommodate the block files. Approximately 810 GB of data will be stored in this directory.

Fix by setting regtest instead of mainnet network before running the test.

Github-Pull: bitcoin#33391
Rebased-From: bdf01c6

Author: ryanofsky

src/test/node_init_tests.cpp

@@ -11,7 +11,12 @@
 
 using node::NodeContext;
 
-BOOST_FIXTURE_TEST_SUITE(node_init_tests, BasicTestingSetup)
+//! Like BasicTestingSetup, but using regtest network instead of mainnet.
+struct InitTestSetup : BasicTestingSetup {
+    InitTestSetup() : BasicTestingSetup{ChainType::REGTEST} {}
+};
+
+BOOST_FIXTURE_TEST_SUITE(node_init_tests, InitTestSetup)
 
 //! Custom implementation of interfaces::Init for testing.
 class TestInit : public interfaces::Init