test: fix some fancy tests after NamedSource change #798

Job	Run time
Check fmt & build docs	29s
Miri	2m 33s
Build & Test (fancy, 1.70.0, ubuntu-latest)	51s
Minimal versions check (ubuntu-latest)	21s
Build & Test (fancy, 1.70.0, macOS-latest)	51s
Minimal versions check (macOS-latest)	59s
Build & Test (fancy, 1.70.0, windows-latest)	36s
Minimal versions check (windows-latest)	45s
Build & Test (fancy, stable, ubuntu-latest)	1s
Build & Test (fancy, stable, macOS-latest)	20s
Build & Test (fancy, stable, windows-latest)	32s
Build & Test (syntect-highlighter, stable, ubuntu-latest)	32s
Build & Test (syntect-highlighter, stable, macOS-latest)	33s
Build & Test (syntect-highlighter, stable, windows-latest)	34s
	9m 57s

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