test: solve data race at TestPutSubmission (plag_routes_test: 86)

[sohlich]

WARNING: DATA RACE
Write by goroutine 31:
github.com/sohlich/go-plag.TestPutSubmission.func1()
/home/radek/Projekty/Go/src/github.com/sohlich/go-plag/plag_routes_test.go:73 +0x30
github.com/sohlich/go-plag.TestPutSubmission()
/home/radek/Projekty/Go/src/github.com/sohlich/go-plag/plag_routes_test.go:87 +0x4ec
testing.tRunner()
/tmp/workdir/go/src/testing/testing.go:456 +0xdc

Previous read by goroutine 34:
github.com/sohlich/go-plag.syncWithApac()
/home/radek/Projekty/Go/src/github.com/sohlich/go-plag/plag_apac.go:11 +0x76
github.com/sohlich/go-plag.checkAssignment()
/home/radek/Projekty/Go/src/github.com/sohlich/go-plag/plag_compare.go:107 +0x6f7
github.com/sohlich/go-plag.putSubmission.func1()
/home/radek/Projekty/Go/src/github.com/sohlich/go-plag/plag_routes.go:70 +0x8a

Goroutine 31 (running) created at:
testing.RunTests()
/tmp/workdir/go/src/testing/testing.go:561 +0xaa3
testing.(*M).Run()
/tmp/workdir/go/src/testing/testing.go:494 +0xe4
main.main()
github.com/sohlich/go-plag/_test/_testmain.go:86 +0x20f

Goroutine 34 (finished) created at:
github.com/sohlich/go-plag.putSubmission()
/home/radek/Projekty/Go/src/github.com/sohlich/go-plag/plag_routes.go:76 +0x9ec
github.com/gin-gonic/gin.(_Context).Next()
/home/radek/Projekty/Go/src/github.com/gin-gonic/gin/context.go:95 +0x145
github.com/gin-gonic/gin.(_Engine).handleHTTPRequest()
/home/radek/Projekty/Go/src/github.com/gin-gonic/gin/gin.go:294 +0x405
github.com/gin-gonic/gin.(*Engine).ServeHTTP()
/home/radek/Projekty/Go/src/github.com/gin-gonic/gin/gin.go:275 +0x2e8
github.com/sohlich/go-plag.TestPutSubmission()
/home/radek/Projekty/Go/src/github.com/sohlich/go-plag/plag_routes_test.go:82 +0x3da
testing.tRunner()
/tmp/workdir/go/src/testing/testing.go:456 +0xdc