Use `assert_prism_eval` over `test_prism_eval` in helper

When we use `test_prism_eval`, failed tests will point to the line
number of the `test_prism_eval` method definition instead of the test
that failed. If we use `assert_prism_eval` instead, failed tests will
properly poin to the test that failed because the test framework knows
to stop the backtrace earlier.

Before line number in failure points to helper definition:

```
Prism::TestCompilePrism#test_RegularExpressionNode = 0.21 s
  1) Failure:
Prism::TestCompilePrism#test_RegularExpressionNode [test/ruby/test_compile_prism.rb:755]:
</pit/> (Windows-31J) expected but was
</pit/> (US-ASCII).
```

After line number in failure points to test that failed:

```
Prism::TestCompilePrism#test_RegularExpressionNode = 0.00 s
  1) Failure:
Prism::TestCompilePrism#test_RegularExpressionNode [test/ruby/test_compile_prism.rb:334]:
</pit/> (Windows-31J) expected but was
</pit/> (US-ASCII).
```

0 files changed, 0 insertions, 0 deletions