use getBy query instead of queryBy when asserting for elements present in document (#2951)

* replace queryByText with getByText when asserting for presence of element

* use string instead of regex for line numbers

* cleanup test case of CodeSnippet

.changeset/lovely-suits-flash.md

@@ -0,0 +1,5 @@
+---
+'@backstage/core': patch
+---
+
+update the test cases of CodeSnippet component