Fix unit tests

[csummers25]

• Fixes the broken test caused by the JustRelax.Mask type not implementing the copy function, by adding a copy definition for this type.
• Adds a test of this new copy function.
• Adds a type check test for the mask_vbox fields in the Stokes object.

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diff --git a/test/test_mask.jl b/test/test_mask.jl
index 4cbcc9e..5ffce59 100644
--- a/test/test_mask.jl
+++ b/test/test_mask.jl
@@ -21,7 +21,7 @@ import JustRelax.JustRelax3D as JR3
     @test all(isone, m[4:7, 4:7])
 
     # test copy
-    @test all(copy(m)[1:ni[1],1:ni[2]].== m[1:ni[1],1:ni[2]])
+    @test all(copy(m)[1:ni[1], 1:ni[2]] .== m[1:ni[1], 1:ni[2]])
 
     # test masking
     A = rand(ni...)
@@ -64,7 +64,7 @@ end
     @test all(isone, m[4:7, 4:7, 4:7])
 
     # test copy
-    @test all(copy(m)[1:ni[1],1:ni[2],1:ni[3]].== m[1:ni[1],1:ni[2],1:ni[3]])
+    @test all(copy(m)[1:ni[1], 1:ni[2], 1:ni[3]] .== m[1:ni[1], 1:ni[2], 1:ni[3]])
 
     A = rand(ni...)
     B = zeros(ni...)