Genericize TestExpression and make value abstract

This seems to resolve issues with the compiler making unions of
unrelated test case types.

src/spectator/expectations/expectation_partial.cr

@@ -6,16 +6,16 @@ module Spectator::Expectations
   # Stores part of an expectation (obviously).
   # The part of the expectation this type covers is the actual value and source.
   # This can also cover a block's behavior.
-  struct ExpectationPartial
+  struct ExpectationPartial(T)
     # The actual value being tested.
     # This also contains its label.
-    getter actual : TestExpression
+    getter actual : TestExpression(T)
 
     # Location where this expectation was defined.
     getter source : Source
 
     # Creates the partial.
-    def initialize(@actual, @source)
+    def initialize(@actual : TestExpression(T), @source : Source)
     end
 
     # Asserts that some criteria defined by the matcher is satisfied.

src/spectator/test_block.cr

@@ -2,7 +2,7 @@ require "./test_expression"
 
 module Spectator
   # Captures an block from a test and its label.
-  struct TestBlock(ReturnType) < TestExpression
+  struct TestBlock(ReturnType) < TestExpression(ReturnType)
     # Calls the block and retrieves the value.
     def value : ReturnType
       @proc.call

src/spectator/test_expression.cr

@@ -1,6 +1,6 @@
 module Spectator
   # Base type for capturing an expression from a test.
-  abstract struct TestExpression
+  abstract struct TestExpression(T)
     # User-friendly string displayed for the actual expression being tested.
     # For instance, in the expectation:
     # ```
@@ -15,6 +15,8 @@ module Spectator
     def initialize(@label)
     end
 
+    abstract def value : T
+
     # String representation of the expression.
     def to_s(io)
       io << label

src/spectator/test_value.cr

@@ -2,7 +2,7 @@ require "./test_expression"
 
 module Spectator
   # Captures a value from a test and its label.
-  struct TestValue(T) < TestExpression
+  struct TestValue(T) < TestExpression(T)
     # Actual value.
     getter value : T