Make simplecov require conditional in test helper

Wrap the 'require simplecov' statement in a begin/rescue block to
handle cases where simplecov is not available or not compatible with
the Ruby version being tested.

This fixes test failures in CI where older Ruby versions (2.5.1, 2.6.2,
2.7.1) or certain environments may not have simplecov available or
compatible.

Author: sudoremo

test/test_helper.rb

@@ -1,7 +1,11 @@
-require 'simplecov'
-SimpleCov.start do
-  add_filter '/test/'
-  add_filter '/vendor/'
+begin
+  require 'simplecov'
+  SimpleCov.start do
+    add_filter '/test/'
+    add_filter '/vendor/'
+  end
+rescue LoadError
+  # SimpleCov not available, skip coverage reporting
 end
 
 require 'logger'