Add syntax tests for ignored filename suffixes

tests/syntax-tests/source/Ignored suffixes/test.rs.bak

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+// foo.bak (editor etc backup) should highlight same as foo

tests/syntax-tests/source/Ignored suffixes/test.rs.dpkg-dist

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+// foo.dpkg-dist (Debian dpkg backup) should highlight same as foo

tests/syntax-tests/source/Ignored suffixes/test.rs.dpkg-old

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+// foo.dpkg-old (Debian dpkg backup) should highlight same as foo

tests/syntax-tests/source/Ignored suffixes/test.rs.in

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+// foo.in (build system input) should highlight same as foo

tests/syntax-tests/source/Ignored suffixes/test.rs.in.in

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+// foo.in.in (build system input, doubly replaced) should highlight same as foo

tests/syntax-tests/source/Ignored suffixes/test.rs.old

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+// foo.old (editor etc backup) should highlight same as foo

tests/syntax-tests/source/Ignored suffixes/test.rs.orig

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+// foo.orig (editor, diff etc backup) should highlight same as foo

tests/syntax-tests/source/Ignored suffixes/test.rs.orig~

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+// foo.orig~ (backup of an editor, diff etc backup) should highlight same as foo

tests/syntax-tests/source/Ignored suffixes/test.rs.rpmnew

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+// foo.rpmnew (Red Hat rpm backup) should highlight same as foo

tests/syntax-tests/source/Ignored suffixes/test.rs.rpmorig

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+// foo.rpmorig (Red Hat rpm backup) should highlight same as foo

tests/syntax-tests/source/Ignored suffixes/test.rs.rpmsave

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+// foo.rpmsave (Red Hat rpm backup) should highlight same as foo

tests/syntax-tests/source/Ignored suffixes/test.rs~

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+// foo~ (editor backup) should highlight same as foo

tests/syntax-tests/source/Ignored suffixes/test.unknown~

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+// foo~ for unknown foo should not highlight