test: test if @mtkmodel works with fully-qualified names

Refactor out `Pin` and `Ground` into a mock-module `MyMockModule`, so
we can use `MyMockModule` and see if `@mtkmodel` works with
components that have fully-qualified names (i.e., `MyMockModule.Ground`).

test/model_parsing.jl

@@ -7,6 +7,30 @@ using Unitful
 
 ENV["MTK_ICONS_DIR"] = "$(@__DIR__)/icons"
 
+# Mock module used to test if the `@mtkmodel` macro works with fully-qualified names as well.
+module MyMockModule
+    using ..ModelingToolkit, Unitful
+
+    export Pin
+    @connector Pin begin
+        v(t), [unit = u"V"]                    # Potential at the pin [V]
+        i(t), [connect = Flow, unit = u"A"]    # Current flowing into the pin [A]
+        @icon "pin.png"
+    end
+
+    @mtkmodel Ground begin
+        @components begin
+            g = Pin()
+        end
+        @icon read(abspath(ENV["MTK_ICONS_DIR"], "ground.svg"), String)
+        @equations begin
+            g.v ~ 0
+        end
+    end
+end
+
+using .MyMockModule
+
 @connector RealInput begin
     u(t), [input = true, unit = u"V"]
 end
@@ -28,12 +52,6 @@ end
 @variables t [unit = u"s"]
 D = Differential(t)
 
-@connector Pin begin
-    v(t), [unit = u"V"]                    # Potential at the pin [V]
-    i(t), [connect = Flow, unit = u"A"]    # Current flowing into the pin [A]
-    @icon "pin.png"
-end
-
 @named p = Pin(; v = π)
 @test getdefault(p.v) == π
 @test Pin.isconnector == true
@@ -57,16 +75,6 @@ end
 
 @test OnePort.isconnector == false
 
-@mtkmodel Ground begin
-    @components begin
-        g = Pin()
-    end
-    @icon read(abspath(ENV["MTK_ICONS_DIR"], "ground.svg"), String)
-    @equations begin
-        g.v ~ 0
-    end
-end
-
 resistor_log = "$(@__DIR__)/logo/resistor.svg"
 @mtkmodel Resistor begin
     @extend v, i = oneport = OnePort()
@@ -127,7 +135,7 @@ end
         capacitor = Capacitor(; C = C_val)
         source = Voltage()
         constant = Constant(; k = k_val)
-        ground = Ground()
+        ground = MyMockModule.Ground()
     end
 
     @equations begin