Improve the completion tests a bit (#228)

Author: natecook1000

Tests/ArgumentParserExampleTests/MathExampleTests.swift

@@ -185,6 +185,31 @@ extension MathExampleTests {
       command: "math --generate-completion-script zsh",
       expected: zshCompletionScriptText)
   }
+  
+  func testMath_CustomCompletion() {
+    AssertExecuteCommand(
+      command: "math ---completion stats quantiles -- --custom",
+      expected: """
+        hello
+        helicopter
+        heliotrope
+        """)
+    
+    AssertExecuteCommand(
+      command: "math ---completion stats quantiles -- --custom h",
+      expected: """
+        hello
+        helicopter
+        heliotrope
+        """)
+  
+    AssertExecuteCommand(
+      command: "math ---completion stats quantiles -- --custom a",
+      expected: """
+        aardvark
+        aaaaalbert
+        """)
+  }
 }
 
 private let bashCompletionScriptText = """

Tests/ArgumentParserUnitTests/CompletionScriptTests.swift

@@ -116,16 +116,7 @@ extension CompletionScriptTests {
   }
 
   func testEscaped_Zsh() throws {
-    let script1 = try CompletionsGenerator(command: Escaped.self, shell: .zsh)
-          .generateCompletionScript()
-    XCTAssertEqual(zshEscapedCompletion, script1)
-
-    let script2 = try CompletionsGenerator(command: Escaped.self, shellName: "zsh")
-          .generateCompletionScript()
-    XCTAssertEqual(zshEscapedCompletion, script2)
-
-    let script3 = Escaped.completionScript(for: .zsh)
-    XCTAssertEqual(zshEscapedCompletion, script3)
+    XCTAssertEqual(zshEscapedCompletion, Escaped.completionScript(for: .zsh))
   }
 }