Performance test

This loads in a truth & run VCF from environment variables. It constructs an ExaminePage and then selects two records.

I chose not to add the VCF files to source control since they're fairly large.

Sample output:
```
$ ./scripts/perf-test.sh
Found 1 matching test...
 PASS  __tests__/ExaminePage-perf.js (17.416s)
Starting timer...
1638ms, 1638ms lap: Parsed run VCF
3106ms, 1468ms lap: Parsed truth VCF
4649ms, 1543ms lap: Constructed <ExaminePage/>
4650ms, 1ms lap: Selected first record
4651ms, 1ms lap: Selected 99th record
1 test passed (1 total)
Run time: 19.468s
./scripts/perf-test.sh  12.87s user 1.24s system 69% cpu 20.379 total
```