Rollup merge of #66369 - tmiasko:compiletest-stamp, r=Mark-Simulacrum
Rollup merge of #66369 - tmiasko:compiletest-stamp, r=Mark-Simulacrum

compiletest: Obtain timestamps for common inputs only once

Obtain timestamps for common inputs (e.g., libraries in run-lib path, or
sources in `src/tool/compiletest/`) only once and reuse the result,
instead of repeating the work for each test case.

graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust