Fix Go test suite #3175
Description
Currently the Go target has some problems that prevented from passing tests. @Joakker and @prnvbn have agreed to help with the Go target so let's see if we can fix that first and then come back to #2504
Here's a link to the failing appveyor for Go:
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T E S T S
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Running org.antlr.v4.test.runtime.go.TestCompositeLexers
Tests run: 1, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 0.406 sec <<< FAILURE! - in org.antlr.v4.test.runtime.go.TestCompositeLexers
org.antlr.v4.test.runtime.go.TestCompositeLexers Time elapsed: 0.406 sec <<< ERROR!
java.lang.Exception:
Non-zero exit while caching go runtime, output: go: go.mod file not found in current directory or any parent directory; see 'go help modules'
at org.antlr.v4.test.runtime.go.TestCompositeLexers.groupSetUp(TestCompositeLexers.java:24)
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Seems go.mod is missing whatever that is!