internal/tracecontext: removes t.Run from tests
internal/tracecontext: removes t.Run from tests

t.Run does not yet exist in 1.6. Fixes build.

Change-Id: I1c653baedbc5a9543bb8a2dc4b7bde0def286632
Reviewed-on: https://code-review.googlesource.com/13530
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go