refactor: rename files to follow Go convention

testdata/error_strings_with_custom_functions.go

@@ -0,0 +1,12 @@
+package fixtures
+
+import (
+	pkgErrors "github.com/pkg/errors"
+)
+
+// Check for the error strings themselves.
+
+func errorsStrings(x int) error {
+	var err error
+	return pkgErrors.Wrap(err, "This %d is too low") // MATCH /error strings should not be capitalized or end with punctuation or a newline/
+}