Unstable test case TestConvergenceAndAccuracy

[tonyyang-svail]

TestConvergenceAndAccuracy fails occasionally on our CI.

Build log: https://travis-ci.com/sql-machine-learning/sqlflow/jobs/277786322

--- FAIL: TestConvergenceAndAccuracy (20.18s)
    convergence_test.go:76: 
        	Error Trace:	convergence_test.go:76
        	            				convergence_test.go:56
        	Error:      	Not equal: 
        	            	expected: 1
        	            	actual  : 0
        	Test:       	TestConvergenceAndAccuracy
    convergence_test.go:76: 
        	Error Trace:	convergence_test.go:76
        	            				convergence_test.go:56
        	Error:      	Not equal: 
        	            	expected: 0
        	            	actual  : 1
        	Test:       	TestConvergenceAndAccuracy
    convergence_test.go:76: 
        	Error Trace:	convergence_test.go:76
        	            				convergence_test.go:56
        	Error:      	Not equal: 
        	            	expected: 0
        	            	actual  : 1
        	Test:       	TestConvergenceAndAccuracy
    convergence_test.go:76: 
        	Error Trace:	convergence_test.go:76
        	            				convergence_test.go:56
        	Error:      	Not equal: 
        	            	expected: 0
        	            	actual  : 1
        	Test:       	TestConvergenceAndAccuracy
    convergence_test.go:76: 
        	Error Trace:	convergence_test.go:76
        	            				convergence_test.go:56
        	Error:      	Not equal: 
        	            	expected: 1
        	            	actual  : 0
        	Test:       	TestConvergenceAndAccuracy
    convergence_test.go:76: 
        	Error Trace:	convergence_test.go:76
        	            				convergence_test.go:56
        	Error:      	Not equal: 
        	            	expected: 1
        	            	actual  : 2
        	Test:       	TestConvergenceAndAccuracy