Change the "write-tests" task prompt to enforce the generated code to…

… be inside code fences, to ensure we can extract the code from the LLMs response

Part of #257

model/llm/llm.go

@@ -77,7 +77,7 @@ type llmSourceFilePromptContext struct {
 var llmGenerateTestForFilePromptTemplate = template.Must(template.New("model-llm-generate-test-for-file-prompt").Parse(bytesutil.StringTrimIndentations(`
 	Given the following {{ .Language.Name }} code file "{{ .FilePath }}" with package "{{ .ImportPath }}", provide a test file for this code{{ with $testFramework := .Language.TestFramework }} with {{ $testFramework }} as a test framework{{ end }}.
 	The tests should produce 100 percent code coverage and must compile.
-	The response must contain only the test code and nothing else.
+	The response must contain only the test code in a fenced code block and nothing else.
 
 	` + "```" + `{{ .Language.ID }}
 	{{ .Code }}