Add tests for new parameter expansion features

tests/parameter_expansion.in

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+# basic expansion test
+echo {}
+echo {apple}
+echo {apple,orange}
+
+# expansion tests with spaces
+echo {apple, orange}
+echo { apple, orange, banana }
+
+# expansion with spaces and cartesian products
+echo \'{ hello , world }\'
+
+# expansion with escapes
+for phrase in {good\,,   beautiful ,morning}; echo -n "$phrase "; end | string trim;
+for phrase in {goodbye\,,\ cruel\ ,world\n}; echo -n $phrase; end;
+
+# whitespace within entries converted to spaces in a single entry
+for foo in { hello
+world }
+	echo \'$foo\'
+end
+
+# dual expansion cartesian product
+echo { alpha, beta }\ {lambda, gamma }, | sed -r 's/(.*),/\1/'
+
+# expansion with subshells
+for name in { (echo Meg), (echo Jo) }
+	echo $name
+end
+
+# subshells with expansion
+for name in (for name in {Beth, Amy}; printf "$name\n"; end); printf "$name\n"; end
+
+# vim: set ft=fish:

tests/parameter_expansion.out

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+{}
+apple
+apple orange
+apple orange
+apple orange banana
+'hello' 'world'
+good, beautiful morning
+goodbye, cruel world
+'hello world'
+alpha lambda, beta lambda, alpha gamma, beta gamma
+Meg
+Jo
+Beth
+Amy