REGTEST: Reg tests file renaming.

Remove old s_ prefixes for some reg tests after they have been flagged
as slow by 91704bfa commit (REGTEST: Flag some slow reg tests).
diff --git a/reg-tests/peers/s_basic_sync_wo_stkt_backend.vtc b/reg-tests/peers/basic_sync_wo_stkt_backend.vtc
similarity index 100%
rename from reg-tests/peers/s_basic_sync_wo_stkt_backend.vtc
rename to reg-tests/peers/basic_sync_wo_stkt_backend.vtc
diff --git a/reg-tests/peers/s_tls_basic_sync_wo_stkt_backend.vtc b/reg-tests/peers/tls_basic_sync_wo_stkt_backend.vtc
similarity index 100%
rename from reg-tests/peers/s_tls_basic_sync_wo_stkt_backend.vtc
rename to reg-tests/peers/tls_basic_sync_wo_stkt_backend.vtc