Make recording package concurrency safe

[chlowell]

Live tests would run significantly faster if they executed test cases in parallel. That isn't possible today because our recording infrastructure isn't concurrency safe. Making it so would require at least synchronizing testSuite access (both reads and writes):

azure-sdk-for-go/sdk/internal/recording/recording.go

Line 515 in caf1a9c

var testSuite = map[string]recordedTest{}