Fix tests and run tests on ci

[carole-lavillonniere]

1. Fixed the failing unit tests and type check:

--- FAIL: TestResponseTooLarge (0.01s)
    invocationresponse_test.go:67:
        	Error Trace:	/home/carolavillo/Workspaces/localstack/lambda-runtime-init/lambda/rapi/handler/invocationresponse_test.go:67
        	Error:      	Not equal:
        	            	expected: "Response payload size (6291557 bytes) exceeded maximum allowed payload size (6291556 bytes)."
        	            	actual  : "Response payload size exceeded maximum allowed payload size (6291556 bytes)."
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1 +1 @@
        	            	-Response payload size (6291557 bytes) exceeded maximum allowed payload size (6291556 bytes).
        	            	+Response payload size exceeded maximum allowed payload size (6291556 bytes).
        	Test:       	TestResponseTooLarge

lambda/core/directinvoke/customerheaders.go:1: : # go.amzn.com/lambda/core/directinvoke [go.amzn.com/lambda/core/directinvoke.test]
lambda/core/directinvoke/directinvoke_test.go:122:26: cannot use interop.MaxPayloadSize (variable of type int) as int64 value in assignment
lambda/core/directinvoke/directinvoke_test.go:141:26: cannot use interop.MaxPayloadSize (variable of type int) as int64 value in assignment
lambda/core/directinvoke/directinvoke_test.go:166:26: cannot use interop.MaxPayloadSize (variable of type int) as int64 value in assignment
lambda/core/directinvoke/directinvoke_test.go:186:26: cannot use interop.MaxPayloadSize (variable of type int) as int64 value in assignment
lambda/core/directinvoke/directinvoke_test.go:278:26: cannot use interop.MaxPayloadSize (variable of type int) as int64 value in assignment
lambda/core/directinvoke/directinvoke_test.go:313:26: cannot use interop.MaxPayloadSize (variable of type int) as int64 value in assignment (typecheck)

2. Added Github job to run unit tests