[gdb/testsuite] Fix hello.go xpass

[gdb/testsuite] Fix hello.go xpass

With gdb.go/hello.go, we run into an xpass:
...
Thread 1 "hello" hit Breakpoint 1, main.main () at hello.go:7^M
7       func main () {^M
(gdb) print st^M
$1 = 0x0 ""^M
(gdb) XPASS: gdb.go/hello.exp: starting string check
...

The xfail is setup as follows:
...
\# This used to print "", i.e., the local "st" initialized as "".
setup_xfail "*-*-*"

gdb_test "print st" \
    ".* = $hex \"\"" \
    "starting string check"
...

It's not clear what gccgo/gc PR this xfail refers to.

It's also not clear why the empty string is both:
- listed as reason for xfail, and
- used in the pass pattern.

Furthermore, there's a comment in the hello.go testcase:
...
  st := "Hello, world!" // this intentionally shadows the global "st"
...
while there's no global st variable present, only a variable myst:
...
var myst = "Shall we?"
...

Fix this by splitting up the test-case in two test-cases, hello.{go,exp} and
global-local-var-shadow.{go,exp}.

In hello.exp we no longer attempt to print st before its declaration.  In
hello.go we remove the myst variable as well the comment related to shadowing.

In global-local-var-shadow.go, we rename myst to st, such that the comment
related to shadowing is correct.  In global-local-var-shadow.exp we attempt to
print the value of st before the local definition, which should print the
value of the global definition, and xfail this with reference to GCC PR93844.

Tested on x86_64-linux, with gccgo 10.

gdb/testsuite/ChangeLog:

2020-02-20  Tom de Vries  <tdevries@suse.de>

	PR go/17018
	* gdb.go/hello.exp: Copy ...
	* gdb.go/global-local-var-shadow.exp: ... here.  New file.  Expect
	print of st to print value of global definition. Add xfail for GCC
	PR93844.
	* gdb.go/hello.exp: Remove printing of st before definition.
	* gdb.go/hello.go: Copy ...
	* gdb.go/global-local-var-shadow.go: ... here. New test.  Rename myst
	to st.
	* gdb.go/hello.go: Remove myst.  Remove comment about shadowing.