test: rename defaults kwarg to initial_conditions

The v3 System constructor renamed `defaults` to `initial_conditions`.
Passing `defaults =` is silently dropped, so the previous commits did
not actually anchor the mass states.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>

Author: ChrisRackauckas

test/Hydraulic/isothermal_compressible.jl

@@ -166,9 +166,9 @@ end
             mass.v ~ 0.0
         ]
 
-        defaults = [mass.s => 0.0, mass.v => 0.0]
+        initial_conditions = [mass.s => 0.0, mass.v => 0.0]
 
-        System(eqs, t, [], pars; name, systems, initialization_eqs, defaults)
+        System(eqs, t, [], pars; name, systems, initialization_eqs, initial_conditions)
     end
 
     @named sys = TestSystem()
@@ -303,14 +303,14 @@ end
             # body.s ~ 0
         ]
 
-        defaults = [
+        initial_conditions = [
             piston.mass.s => 0.0,
             piston.mass.v => 0.0,
             valve.mass.s => 0.0,
             valve.mass.v => 0.0,
         ]
 
-        System(eqs, t, vars, pars; name, systems, initialization_eqs, defaults)
+        System(eqs, t, vars, pars; name, systems, initialization_eqs, initial_conditions)
     end
 
     @mtkcompile initsys = ActuatorSystem(false)
@@ -374,9 +374,9 @@ end
             mass.v ~ 0
         ]
 
-        defaults = [mass.s => 0.05, mass.v => 0.0]
+        initial_conditions = [mass.s => 0.05, mass.v => 0.0]
 
-        return System(eqs, t, [], pars; name, systems, initialization_eqs, defaults)
+        return System(eqs, t, [], pars; name, systems, initialization_eqs, initial_conditions)
     end
 
     @mtkcompile sys = HydraulicSystem()