[gdb/testsuite] Allow debug srcfile2 in gdb.base/unwind-on-each-insn.exp

[gdb/testsuite] Allow debug srcfile2 in gdb.base/unwind-on-each-insn.exp

Test-case gdb.base/unwind-on-each-insn.exp compiles $srcfile with debug info, and
$srcfile2 without.

Occasionally, I try both files with debug info:
...
-             $srcfile $debug_flags $srcfile2 $nodebug_flags]]} {
+             $srcfile $debug_flags $srcfile2 $debug_flags]]} {
...

This shortcuts the test due to no longer recognizing that stepi still lands
in foo.

Fix this by determining whether still in foo by checking the info frame output.

Tested on x86_64-linux.