Fix prompt cache flaky test

[tompng]

"#{rand}" might include "e-" which doesn't match to [\d.]+

Fix this failed flaky test

Failure: test_prompt_part_cached(TestIRB::ContextTest):
  </\A\[(to_s[\d.]+) (inspect[\d.]+) \1 \2\]>\z/> was expected to be =~
  <"[to_s0.34254925528742264 inspect4.070655031584991e-05 to_s0.34254925528742264 inspect4.070655031584991e-05]>">.