Enable precompiled header for tests

[saki7]

fixes #37

Total build

Before

========== Build: 1 succeeded, 0 failed, 68 up-to-date, 6 skipped ==========
========== Build completed at 17:45 and took 10.095 seconds ==========

After

========== Build: 1 succeeded, 0 failed, 68 up-to-date, 6 skipped ==========
========== Build completed at 17:48 and took 07.244 seconds ==========

Single source build (expect.cpp, which is known as a relatively heavy test)

Before

========== Build: 63 succeeded, 0 failed, 7 up-to-date, 6 skipped ==========
========== Build completed at 18:12 and took 30.214 seconds ==========

After

========== Build: 63 succeeded, 0 failed, 7 up-to-date, 6 skipped ==========
========== Build completed at 18:12 and took 26.408 seconds ==========

[cppwarningnotifier]

Environment				C++23	C++26
x4	Clang	21	Debug	✅success	✅success
			Release	✅success	✅success
	GCC	14	Debug	✅success	✅success
			Release	✅success	✅success
	MSVC	2022	Debug	✅success	✅success
			Release	✅success	✅success
		2026	Debug	✅success	✅success
			Release	✅success	✅success