[tests] Workaround Python 2 “narrow” builds

The so-called Python 2 “narrow” builds support UCS2 only, this is a
workaround to allow unichr to work with any Unicode character in such
builds. This fixes Travis-CI failure as it has narrow Python 2 builds.

Copied from:
https://github.com/behdad/fonttools/blob/master/Lib/fontTools/misc/py23.py

test/shaping/hb_test_tools.py

diff --git a/test/shaping/hb_test_tools.py b/test/shaping/hb_test_tools.py
index 747699b..7473982 100644
--- a/test/shaping/hb_test_tools.py
+++ b/test/shaping/hb_test_tools.py
@@ -7,7 +7,43 @@
 diff_symbols = "-+=*&^%$#@!~/"
 diff_colors = ['red', 'green', 'blue']
 
-if sys.version_info[0] >= 3:
+try:
+	unichr = unichr
+
+	if sys.maxunicode < 0x10FFFF:
+		# workarounds for Python 2 "narrow" builds with UCS2-only support.
+
+		_narrow_unichr = unichr
+
+		def unichr(i):
+			"""
+			Return the unicode character whose Unicode code is the integer 'i'.
+			The valid range is 0 to 0x10FFFF inclusive.
+
+			>>> _narrow_unichr(0xFFFF + 1)
+			Traceback (most recent call last):
+			  File "<stdin>", line 1, in ?
+			ValueError: unichr() arg not in range(0x10000) (narrow Python build)
+			>>> unichr(0xFFFF + 1) == u'\U00010000'
+			True
+			>>> unichr(1114111) == u'\U0010FFFF'
+			True
+			>>> unichr(0x10FFFF + 1)
+			Traceback (most recent call last):
+			  File "<stdin>", line 1, in ?
+			ValueError: unichr() arg not in range(0x110000)
+			"""
+			try:
+				return _narrow_unichr(i)
+			except ValueError:
+				try:
+					padded_hex_str = hex(i)[2:].zfill(8)
+					escape_str = "\\U" + padded_hex_str
+					return escape_str.decode("unicode-escape")
+				except UnicodeDecodeError:
+					raise ValueError('unichr() arg not in range(0x110000)')
+
+except NameError:
 	unichr = chr
 
 class ColorFormatter: