test: fix test case

Issue introduced by https://go-review.googlesource.com/#/c/16920/ .

TBR=rsc

Change-Id: I2a0e0c81f641f869568230837c566913f6538f37
Reviewed-on: https://go-review.googlesource.com/16990
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

test/fixedbugs/issue11590.go

@@ -6,6 +6,6 @@
 
 package p
 
-var _ = int8(4) * 300         // ERROR "constant overflows int8"
-var _ = complex64(1) * 1e200  // ERROR "constant overflows complex64"
-var _ = complex128(1) * 1e500 // ERROR "constant overflows complex128"
+var _ = int8(4) * 300         // ERROR "constant 300 overflows int8" "constant 1200 overflows int8"
+var _ = complex64(1) * 1e200  // ERROR "constant 1e\+200 overflows complex64"
+var _ = complex128(1) * 1e500 // ERROR "constant 1\.00000e\+500 overflows complex128"