go/ssa: implement missing testing.testDeps SetPanicOnExit0 method

A new method was added to testing.testDeps in CL 250977, which causes
the implementation of that (unexported, explicitly-unstable) interface
in go/ssa to no longer satisfy the interface.

This is a quick, short-term fix. Longer term, the go/ssa package
should switch to the supported TestMain API.

Updates golang/go#41186

Change-Id: Ice110ef540e31a0a98085713346126ffeae64d5e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/252677
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/go/ssa/testmain.go b/go/ssa/testmain.go
index 4bf8d98..c4256d1 100644
--- a/go/ssa/testmain.go
+++ b/go/ssa/testmain.go
@@ -222,6 +222,7 @@
 func (deps) ImportPath() string { return "" }
 func (deps) MatchString(pat, str string) (bool, error) { return true, nil }
+func (deps) SetPanicOnExit0(bool) {}
 func (deps) StartCPUProfile(io.Writer) error { return nil }
 func (deps) StartTestLog(io.Writer) {}
 func (deps) StopCPUProfile() {}