Avoid head -c GNUism in reftests #2906

Job	Run time
Analyse	4s
Build-macOS (4.14.1)	2m 32s
Hygiene	14s
Cygwin	4s
Build-Linux (4.08.1)	1m 54s
Build-Linux (4.09.1)	1m 56s
Test-macOS (4.14.1)	27m 4s
Build-Linux (4.10.2)	2m 34s
Build-Linux (4.11.2)	1m 58s
Build-Linux (4.12.1)	2m 18s
Build-Linux (4.13.1)	2m 8s
Build-Linux (5.0.0)	1m 56s
Build-Linux (5.1.1)	1m 54s
Build-Linux (4.14.1)	1m 50s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.1)	38m 23s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	47m 25s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	49m 23s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.1)	51m 40s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.1)	52m 57s
Upgrade-macOS (4.14.1)	1m 50s
Solvers-macOS (z3, 4.14.1)	1m 21s
Solvers-macOS (0install, 4.14.1)	1m 14s
Cold-Linux	4m 49s
Solvers-Linux (z3, 4.14.1)	50s
Test-Linux (4.14.1)	13m 27s
Solvers-Linux (0install, 4.14.1)	1m 27s
Upgrade-Linux (4.14.1)	1m 10s
	5h 14m 22s

Provide feedback