playground: fix panic if failed to start tiflash

[july2993]

Fix #538

We always Wait the instances before, if it failed to start it will panic caused the Command is not initialized and started.

This PR make sure it only Wait the instance iff it's started successfully.

[codecov-commenter]

Codecov Report

Merging #543 into master will increase coverage by 11.14%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #543       +/-   ##
===========================================
+ Coverage   39.84%   50.98%   +11.14%     
===========================================
  Files         201      220       +19     
  Lines       14847    15914     +1067     
===========================================
+ Hits         5916     8114     +2198     
+ Misses       8072     6693     -1379     
- Partials      859     1107      +248     

Flag	Coverage Δ
#coverage	50.98% <ø> (+11.14%)	⬆️

Impacted Files	Coverage Δ
...o/src/github.com/pingcap/tiup/server/store/sync.go	28.57% <0.00%> (-28.58%)	⬇️
....com/pingcap/tiup/pkg/cluster/clusterutil/retry.go	50.00% <0.00%> (-17.50%)	⬇️
go/src/github.com/pingcap/tiup/cmd/mirror.go	39.95% <0.00%> (-6.89%)	⬇️
...ingcap/tiup/components/playground/instance/tidb.go	85.36% <0.00%> (-5.34%)	⬇️
...ingcap/tiup/components/playground/instance/tikv.go	75.00% <0.00%> (-4.32%)	⬇️
go/src/github.com/pingcap/tiup/cmd/root.go	60.43% <0.00%> (-3.85%)	⬇️
...cap/tiup/components/playground/instance/tiflash.go	61.29% <0.00%> (-2.49%)	⬇️
...om/pingcap/tiup/components/cluster/command/root.go	43.03% <0.00%> (-2.24%)	⬇️
.../pingcap/tiup/components/playground/instance/pd.go	75.43% <0.00%> (-1.23%)	⬇️
...pingcap/tiup/components/cluster/command/destroy.go	62.00% <0.00%> (-1.16%)	⬇️
... and 116 more

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[july2993]

@lonng PTAL

[lonng]

LGTM

[lonng]

/merge