fix!: Remove unstable features from libtest

[epage]

This originally started as a direct replacement for libtest and was aiming for 1:1 feature parity, even for unstable features.

As we've instead shifted to trying to define a minimal API for test runners to call into and for a lot of these responsibilities to be in test runners, a lot of this does not make sense to include.

[coveralls]

Pull Request Test Coverage Report for Build 16205816091

Details

• 0 of 2 (0.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+4.8%) to 33.333%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/libtest-lexarg/src/lib.rs	0	1	0.0%
crates/libtest2-harness/src/notify/summary.rs	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
crates/libtest2-harness/src/notify/summary.rs	1	0.0%

Totals
Change from base Build 16205277566:	4.8%
Covered Lines:	326
Relevant Lines:	978

---

💛 - Coveralls