Avoid head -c GNUism in reftests #2904

Job	Run time
Analyse	5s
Cygwin	7s
Hygiene	14s
Build-macOS (4.14.1)	2m 19s
Build-Linux (4.08.1)	1m 47s
Test-macOS (4.14.1)	34m 53s
Build-Linux (4.09.1)	2m 2s
Build-Linux (4.10.2)	1m 52s
Build-Linux (4.11.2)	1m 50s
Build-Linux (4.12.1)	2m 1s
Build-Linux (4.13.1)	1m 52s
Build-Linux (5.0.0)	2m 0s
Build-Linux (5.1.1)	1m 47s
Build-Linux (4.14.1)	1m 48s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.1)	38m 35s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	47m 37s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	46m 39s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.1)	53m 37s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.1)	52m 10s
Upgrade-Linux (4.14.1)	56s
Test-Linux (4.14.1)	13m 34s
Cold-Linux	4m 42s
Solvers-Linux (z3, 4.14.1)	50s
Solvers-Linux (0install, 4.14.1)	1m 16s
Upgrade-macOS (4.14.1)	1m 39s
Solvers-macOS (z3, 4.14.1)	1m 19s
Solvers-macOS (0install, 4.14.1)	1m 15s
	5h 18m 46s

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