[libFuzzer] Deflake entropic exec-time test.

Entropic scheduling with exec-time option can be misled, if inputs
on the right path to become crashing inputs accidentally take more
time to execute before it's added to the corpus. This patch, by letting
more of such inputs added to the corpus (four inputs of size 7 to 10,
instead of a single input of size 2), reduces possibilities of being
influenced by timing flakiness.

A longer-term fix could be to reduce timing flakiness in the fuzzer;
one way could be to execute inputs multiple times and take average of
their execution time before they are added to the corpus.

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D113544