test(cmd/gno): prevent nil deref in testMainCaseRun

[n0izn0iz]

In gnovm/cmd/gno/main_test.go, when an error is expected but none happen, the test helper tries to dereference nil, causing a confusing test result

Before:

=== RUN   TestModApp/mod_tidy..~..~tests~integ~invalid_module_version1
    main_test.go:92: recover runtime error: invalid memory address or nil pointer dereference
    main_test.go:93: 
        	Error Trace:	/Users/norman/Code/gno/gnovm/cmd/gno/main_test.go:93
        	            				/Users/norman/go/pkg/mod/golang.org/[email protected]/src/runtime/panic.go:770
        	            				/Users/norman/go/pkg/mod/golang.org/[email protected]/src/runtime/panic.go:261
        	            				/Users/norman/go/pkg/mod/golang.org/[email protected]/src/runtime/signal_unix.go:881
        	            				/Users/norman/Code/gno/gnovm/cmd/gno/main_test.go:131
        	Error:      	Should be false
        	Test:       	TestModApp/mod_tidy..~..~tests~integ~invalid_module_version1
        	Messages:   	should not panic

After:

=== RUN   TestModApp/mod_tidy..~..~tests~integ~invalid_module_version1
    main_test.go:131: err <nil>
    main_test.go:132: 
        	Error Trace:	/Users/norman/Code/gno/gnovm/cmd/gno/main_test.go:132
        	Error:      	Expected value not to be nil.
        	Test:       	TestModApp/mod_tidy..~..~tests~integ~invalid_module_version1
        	Messages:   	err shouldn't be nil

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.33%. Comparing base (538ebff) to head (d9e93c3).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3071      +/-   ##
==========================================
- Coverage   63.33%   63.33%   -0.01%     
==========================================
  Files         548      548              
  Lines       78646    78646              
==========================================
- Hits        49809    49808       -1     
  Misses      25476    25476              
- Partials     3361     3362       +1     

Flag	Coverage Δ
contribs/gnodev	61.11% <ø> (ø)
contribs/gnofaucet	15.77% <ø> (+0.94%)	⬆️
gno.land	67.18% <ø> (ø)
gnovm	67.88% <ø> (ø)
misc/genstd	79.72% <ø> (ø)
tm2	62.41% <ø> (-0.02%)	⬇️

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