add extra tests for index inserts

[jsteemann]

Addresses https://arangodb.atlassian.net/browse/BTS-468.

Test results from a local run, comparing indexes with selectivity estimates (default) and without selectivity estimates:

.----------------------------------------------------------------------------------------------------------------------------------.
|                                                             indexes                                                              |
|----------------------------------------------------------------------------------------------------------------------------------|
|              testname               | collection | runs | min (s) | max (s) | % dev | avg (s) | stddev (s) | med (s) | total (s) |
|-------------------------------------|------------|------|---------|---------|-------|---------|------------|---------|-----------|
| indexes-insert1-numeric             | 1000k      |    5 |  9.1319 |  9.1879 |  0.61 |  9.1548 |     0.0239 |  9.1663 |   56.7203 |
| indexes-insert1-numeric-noestimates | 1000k      |    5 |  8.7504 |  8.7840 |  0.38 |  8.7618 |     0.0157 |  8.7675 |   54.5821 |
| indexes-insert1-string              | 1000k      |    5 | 10.5819 | 10.6387 |  0.54 | 10.6175 |     0.0254 | 10.6353 |   65.4830 |
| indexes-insert1-string-noestimates  | 1000k      |    5 | 10.1792 | 10.2514 |  0.71 | 10.2265 |     0.0335 | 10.2502 |   63.8927 |
| indexes-insert2-numeric             | 1000k      |    5 | 14.2271 | 14.3386 |  0.78 | 14.2826 |     0.0455 | 14.3103 |   87.7919 |
| indexes-insert2-numeric-noestimates | 1000k      |    5 | 13.2981 | 13.4616 |  1.22 | 13.3631 |     0.0709 | 13.3956 |   82.9297 |
| indexes-insert2-string              | 1000k      |    5 | 17.0645 | 17.2889 |  1.31 | 17.1878 |     0.0930 | 17.2494 |  105.6861 |
| indexes-insert2-string-noestimates  | 1000k      |    5 | 16.6145 | 16.7215 |  0.64 | 16.6694 |     0.0437 | 16.6968 |  102.7363 |
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