systemtests: add heat-exchanger-simplified test suite

Add heat-exchanger-simplified to the system test suites, covering the
fluid-top-openfoam, fluid-btm-openfoam, solid-calculix case combination.

Author: AdityaGupta716

tools/tests/tests.yaml

@@ -128,4 +128,12 @@ test_suites:
           - fluid-openfoam
           - solid-upstream-dealii
           - solid-downstream-dealii
-        reference_result: ./perpendicular-flap/reference-results/fluid-openfoam_solid-upstream-dealii_solid-downstream-dealii.tar.gz
+        reference_result: ./perpendicular-flap/reference-results/fluid-openfoam_solid-upstream-dealii_solid-downstream-dealii.tar.gz
+  heat_exchanger_simplified_test:
+    tutorials:
+      - path: heat-exchanger-simplified
+        case_combination:
+          - fluid-top-openfoam
+          - fluid-btm-openfoam
+          - solid-calculix
+        reference_result: ./heat-exchanger-simplified/reference-results/fluid-top-openfoam_fluid-btm-openfoam_solid-calculix.tar.gz