Fix flaky cache test

[aw-andre]

Fixes a flaky test in runtimes_cache_test.cpp that gives a false failure when the filesystem reuses the inode of a stale cache entry.

Closes #6168

[josh11b]

Removing this will no longer compile.

[CarbonInfraBot]

[diff] _{reported by reviewdog 🐶}

Suggested change

	EXPECT_FALSE(stale_runtimes_0.base_dir().Stat()->unix_inode()
	== stale_runtimes_0_inode &&
	stale_runtimes_0.base_dir().Stat()->mtime()
	== stale_runtimes_0_mtime);
	EXPECT_FALSE(stale_runtimes_0.base_dir().Stat()->unix_inode() ==
	stale_runtimes_0_inode &&
	stale_runtimes_0.base_dir().Stat()->mtime() ==
	stale_runtimes_0_mtime);

[chandlerc]

I don't think this is quite the right way to fix the test. Rather than relying on timestamps, we can force the inode to not be re-used. (I hope.)

I've sent #6387 which does this, and also makes some other improvements to make it easier to debug if this keeps happening.

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