Parallel sweep: add more fiber pool lock assertions to cont.c #12821

Job	Run time
make (check, --disable-yjit)	3m 3s
make (test-bundled-gems)	3m 17s
make (test-bundler-parallel, 50)	2m 53s
make (check, ubuntu-24.04, capi)	3m 10s
make (check, cppflags=-DVM_CHECK_MODE)	3m 24s
make (check, --enable-shared --enable-load-relative)	3m 19s
ruby-bench (--warmup=1 --bench=1 --excludes=shipit)	3m 18s
make (check, i686)	2m 53s
make-ibm	0s
Ubuntu result	3s
	25m 20s

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