Invalid synchronisation in termdash_test

[mum4k]

The tests (specifically the after function) is currently synchronised by waiting for the event queue to empty. This isn't a good synchronisation, since the event processing code in termdash still races with the test code that evaluates the result.

Example traceback:
The command "go test ./..." exited with 0.
12.77s$ go test -race ./...
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x5f1cc8]
goroutine 38 [running]:
testing.tRunner.func1(0xc4200c05a0)
/home/travis/.gimme/versions/go1.10.3.linux.amd64/src/testing/testing.go:742 +0x567
panic(0x62cf60, 0x7881a0)
/home/travis/.gimme/versions/go1.10.3.linux.amd64/src/runtime/panic.go:502 +0x24a
github.com/mum4k/termdash.TestRun.func5(0xc42009c750, 0xc42006f100)
/home/travis/gopath/src/github.com/mum4k/termdash/termdash_test.go:239 +0x68
github.com/mum4k/termdash.TestRun.func11(0xc4200c05a0)
/home/travis/gopath/src/github.com/mum4k/termdash/termdash_test.go:358 +0xa00
testing.tRunner(0xc4200c05a0, 0xc4200833e0)
/home/travis/.gimme/versions/go1.10.3.linux.amd64/src/testing/testing.go:777 +0x16e
created by testing.(*T).Run
/home/travis/.gimme/versions/go1.10.3.linux.amd64/src/testing/testing.go:824 +0x565
FAIL github.com/mum4k/termdash 1.068s