Rearrange tests for `XOnlyPublicKey::from_slice` #1610

Job	Run time
Test - 1.63.0 toolchain (recent)	4m 16s
Docs - stable toolchain (recent)	41s
Test - stable toolchain (recent)	3m 21s
Test - stable toolchain (minimal)	2m 51s
Prepare	3s
Test - 1.63.0 toolchain (minimal)	4m 10s
Test 32-bit version	24s
Format - nightly toolchain	13s
WASM - stable toolchain	1m 37s
Lint - nightly toolchain (recent)	34s
Docs - nightly toolchain (recent)	21s
Test - nightly toolchain (recent)	2m 30s
Test - nightly toolchain (minimal)	2m 25s
Bench - nightly toolchain (recent)	25s
no-std - nightly toolchain (recent)	1m 2s
ASAN - nightly toolchain (recent)	1m 33s
	26m 26s

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