math/big: add test cases for min/max exponent values

Change-Id: I2e74e39628285e2fecaab712be6cff230619a6c2
Reviewed-on: https://go-review.googlesource.com/14778Reviewed-by: Alan Donovan <adonovan@google.com>

src/math/big/floatconv_test.go

@@ -367,10 +367,20 @@ func TestFloatText(t *testing.T) {
 
 		// make sure "stupid" exponents don't stall the machine
 		{"1e1000000", 64, 'p', 0, "0x.88b3a28a05eade3ap+3321929"},
+		{"1e646456992", 64, 'p', 0, "0x.e883a0c5c8c7c42ap+2147483644"},
+		{"1e646456993", 64, 'p', 0, "+Inf"},
 		{"1e1000000000", 64, 'p', 0, "+Inf"},
 		{"1e-1000000", 64, 'p', 0, "0x.efb4542cc8ca418ap-3321928"},
+		{"1e-646456993", 64, 'p', 0, "0x.e17c8956983d9d59p-2147483647"},
+		{"1e-646456994", 64, 'p', 0, "0"},
 		{"1e-1000000000", 64, 'p', 0, "0"},
 
+		// minimum and maximum values
+		{"1p2147483646", 64, 'p', 0, "0x.8p+2147483647"},
+		{"0x.8p2147483647", 64, 'p', 0, "0x.8p+2147483647"},
+		{"0x.8p-2147483647", 64, 'p', 0, "0x.8p-2147483647"},
+		{"1p-2147483649", 64, 'p', 0, "0x.8p-2147483648"},
+
 		// TODO(gri) need tests for actual large Floats
 
 		{"0", 53, 'b', 0, "0"},