test: use the right amount of CGUs in sepcomp-cci-copies to ensure deterministic splitting.

src/test/run-make/sepcomp-cci-copies/Makefile

@@ -2,9 +2,11 @@
 
 # Check that cross-crate inlined items are inlined in all compilation units
 # that refer to them, and not in any other compilation units.
+# Note that we have to pass `-C codegen-units=6` because up to two CGUs may be
+# created for each source module (see `rustc_mir::monomorphize::partitioning`).
 
 all:
 	$(RUSTC) cci_lib.rs
-	$(RUSTC) foo.rs --emit=llvm-ir -C codegen-units=3 \
+	$(RUSTC) foo.rs --emit=llvm-ir -C codegen-units=6 \
 		-Z inline-in-all-cgus
 	[ "$$(cat "$(TMPDIR)"/foo.*.ll | grep -c define\ .*cci_fn)" -eq "2" ]