Fix Go test suite

[parrt]

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)
...

Seems go.mod is missing whatever that is!

[Joakker]

Ok, this one should be pretty easy to implement. go.mod is a signature file that describes the directory and all it's children as part of a module. Recent versions of the go tool have migrated to using only modules by default, so more than likely that's why it's complaining. I'll open a PR shortly with the file added

[prnvbn]

Seems like running go mod init would be enough. Also @Joakker you might have to pull some weight before the 14th of May as I have University exams. After that, I will make up for it!

[Joakker]

Is ok, no worries 😃

[mitar]

Awesome. I can update my PR (#3174) to get tests to pass there, too.

[parrt]

Comments from previous thread: #2504 (comment)