tests: Add avm1/edittext_stylesheet test

This test verifies the behavior of StyleSheet on a TextField in AVM1.

tests/tests/swfs/avm1/edittext_stylesheet/TestFont.sfd

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+SplineFontDB: 3.2
+FontName: TestFont
+FullName: TestFont
+FamilyName: TestFont
+Weight: Regular
+Copyright: Copyright (c) 2024, Kamil Jarosz
+UComments: "2024-7-24: Created with FontForge (http://fontforge.org)"
+Version: 001.000
+ItalicAngle: 0
+UnderlinePosition: -100
+UnderlineWidth: 50
+Ascent: 800
+Descent: 200
+InvalidEm: 0
+LayerCount: 2
+Layer: 0 0 "Back" 1
+Layer: 1 0 "Fore" 0
+XUID: [1021 253 198287149 6396829]
+StyleMap: 0x0000
+FSType: 0
+OS2Version: 0
+OS2_WeightWidthSlopeOnly: 0
+OS2_UseTypoMetrics: 1
+CreationTime: 1721856925
+ModificationTime: 1738521101
+OS2TypoAscent: 0
+OS2TypoAOffset: 1
+OS2TypoDescent: 0
+OS2TypoDOffset: 1
+OS2TypoLinegap: 90
+OS2WinAscent: 0
+OS2WinAOffset: 1
+OS2WinDescent: 0
+OS2WinDOffset: 1
+HheadAscent: 0
+HheadAOffset: 1
+HheadDescent: 0
+HheadDOffset: 1
+OS2Vendor: 'PfEd'
+MarkAttachClasses: 1
+DEI: 91125
+Encoding: ISO8859-1
+UnicodeInterp: none
+NameList: AGL For New Fonts
+DisplaySize: -48
+AntiAlias: 1
+FitToEm: 0
+WinInfo: 0 30 10
+BeginPrivate: 0
+EndPrivate
+BeginChars: 256 12
+
+StartChar: a
+Encoding: 97 97 0
+Width: 800
+Flags: HW
+LayerCount: 2
+Fore
+SplineSet
+0 800 m 1
+ 800 800 l 1
+ 800 0 l 1
+ 0 0 l 1
+ 0 800 l 1
+EndSplineSet
+Validated: 1
+EndChar
+
+StartChar: b
+Encoding: 98 98 1
+Width: 800
+Flags: HW
+LayerCount: 2
+Fore
+SplineSet
+0 0 m 1
+ 800 0 l 1
+ 800 -200 l 1
+ 0 -200 l 1
+ 0 0 l 1
+EndSplineSet
+Validated: 1
+EndChar
+
+StartChar: c
+Encoding: 99 99 2
+Width: 800
+Flags: HW
+LayerCount: 2
+Fore
+SplineSet
+0 800 m 1
+ 800 800 l 1
+ 800 0 l 1
+ 0 0 l 1
+ 0 800 l 1
+EndSplineSet
+EndChar
+
+StartChar: e
+Encoding: 101 101 3
+Width: 1000
+Flags: HW
+LayerCount: 2
+EndChar
+
+StartChar: g
+Encoding: 103 103 4
+Width: 1000
+Flags: HW
+LayerCount: 2
+EndChar
+
+StartChar: i
+Encoding: 105 105 5
+Width: 1000
+Flags: HW
+LayerCount: 2
+EndChar
+
+StartChar: k
+Encoding: 107 107 6
+Width: 1000
+Flags: HW
+LayerCount: 2
+EndChar
+
+StartChar: m
+Encoding: 109 109 7
+Width: 1000
+Flags: HW
+LayerCount: 2
+EndChar
+
+StartChar: o
+Encoding: 111 111 8
+Width: 1000
+Flags: HW
+LayerCount: 2
+EndChar
+
+StartChar: q
+Encoding: 113 113 9
+Width: 1000
+Flags: HW
+LayerCount: 2
+EndChar
+
+StartChar: d
+Encoding: 100 100 10
+Width: 800
+Flags: HW
+LayerCount: 2
+Fore
+SplineSet
+0 0 m 1
+ 800 0 l 1
+ 800 -200 l 1
+ 0 -200 l 1
+ 0 0 l 1
+EndSplineSet
+EndChar
+
+StartChar: space
+Encoding: 32 32 11
+Width: 800
+Flags: HWO
+LayerCount: 2
+EndChar
+EndChars
+EndSplineFont