fluids: Make shocktube test less brittle

- Small, insignificant changes to the coordinate locations make a
  dramatic difference to the initial condition
- For the test case, there is a node right at the midpoint of the
  domain. Adding the epsilon fudge factor makes the if statement more
  stable

examples/fluids/qfunctions/shocktube.h

@@ -94,7 +94,7 @@ CEED_QFUNCTION_HELPER CeedInt Exact_ShockTube(CeedInt dim, CeedScalar time, cons
   CeedScalar rho, P, u[3] = {0.};
 
   // Initial Conditions
-  if (x <= mid_point) {
+  if (x <= mid_point + 200*CEED_EPSILON) {
     rho = rho_high;
     P   = P_high;
   } else {