Fix possible data race in manager/state/store/memory_test.go

[VincentRbbmnd]

This PR is a fix for #2536, essentially using the first proposed way of fixing the original issue.

This fixes a possible data race in manager/state/store/memory_test.go

An alternative to this fix would be to define a new variable in the following way:

    for c := 0; c != 5; c++ {
        ...
        x := c
        go func() {
            ...
            Name: nodeIDs[i%benchmarkNumNodes] + "_" + strconv.Itoa(x) + "_" + strconv.Itoa(i),
            ...
        }
    }

[codecov]

Codecov Report

Merging #2537 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #2537     +/-   ##
========================================
- Coverage   61.49%   61.4%   -0.1%     
========================================
  Files         133     133             
  Lines       21736   21736             
========================================
- Hits        13367   13347     -20     
- Misses       6932    6940      +8     
- Partials     1437    1449     +12

[cyli]

LGTM, thank you for the fix @VincentRbbmnd