Change test data dir_path to be more representative

Related #305 (closed) gitlab-org/gitlab#222685 (closed)

Testing further with SiteSpeed today found it was directly impacted by gitlab-org/gitlab#211366 (closed). The path we're using to test the file lists in both API and Web is massive, containing 1795 files causing a O(n) performance error. This page sequentially loads the files 25 at a time in order and as such takes 2.1 minutes on that directory and breaks SiteSpeed that has a 30 second timeout.

MR switches the dir path we test with GPT to the same one that works in SiteSpeed for consistency and is more representative with around 300 files in it.

* Environment:                Staging
* Environment Version:        13.2.0-pre `3064cb316d7`
* Option:                     60s_200rps
* Date:                       2020-07-16
* Run Time:                   2m 9.13s (Start: 15:49:23 UTC, End: 15:51:32 UTC)
* GPT Version:                v2.0.5

❯ Overall Results Score: 97.84%

NAME                            | RPS   | RPS RESULT           | TTFB AVG | TTFB P90           | REQ STATUS     | RESULT
--------------------------------|-------|----------------------|----------|--------------------|----------------|----------------
api_v4_projects_repository_tree | 200/s | 195.67/s (>160.00/s) | 71.10ms  | 75.73ms (<500ms)   | 100.00% (>95%) | Passed
web_project_files               | 20/s  | 19.55/s (>12.00/s)   | 123.84ms | 196.21ms (<2500ms) | 100.00% (>95%) | Passed