Added a regression test.

Author: Tor Hovland

src/test/run-make/issue-10971-temps-dir/Makefile

@@ -0,0 +1,8 @@
+# Regression test for issue #10971
+# Running two invocations in parallel would overwrite each other's temp files.
+
+all:
+	touch $(TMPDIR)/lib.rs
+
+	$(RUSTC) --crate-type=lib --temps-dir=$(TMPDIR)/temp1 --out-dir=$(TMPDIR) $(TMPDIR)/lib.rs \
+		& $(RUSTC) --crate-type=cdylib --temps-dir=$(TMPDIR)/temp2 --out-dir=$(TMPDIR) $(TMPDIR)/lib.rs