Relax tolerance of test_RelativeGap

[odow]

This failed a recent run of solver-tests.yml in MOI. It's a bit strict regardless, so let's loosen.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (09c0cbb) 83.94% compared to head (945949c) 83.94%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #170 +/- ## ======================================= Coverage 83.94% 83.94% ======================================= Files 3 3 Lines 1607 1607 ======================================= Hits 1349 1349 Misses 258 258 ```

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