[Good first issue] Cover adjuststack instruction with tests

[nvasilevski]

It's a good issue to start contributing!
adjuststack has been implemented but we are missing a test:

tenderjit/lib/tenderjit/iseq_compiler.rb

Lines 2274 to 2276 in b51a3af

	def handle_adjuststack n
	n.times { @temp_stack.pop }
	end

tenderjit/test/instructions/adjuststack_test.rb

Lines 7 to 9 in 4da5234

	def test_adjuststack
	skip "adjuststack has been implemented. Please add a test."
	end

Here is an example of the code that supposed to generate adjuststack instruction:
https://github.com/ruby/ruby/blob/9eae8cdefba61e9e51feb30a4b98525593169666/bootstraptest/test_insns.rb#L133