[gdb/testsuite] Fix gdb.base/exprs.exp for gdb build with byacc

[gdb/testsuite] Fix gdb.base/exprs.exp for gdb build with byacc

On x86_64-freebsd, with test-case gdb.base/exprs.exp I get:
...
(gdb) print 23
yydebug: state 0, reading 257 (INT)
yydebug: state 0, shifting to state 1
yydebug: state 1, reducing by rule 94 (exp : INT)
yydebug: after reduction, shifting from state 0 to state 59
yydebug: state 59, reading 0 (end-of-file)
yydebug: state 59, reducing by rule 7 (exp1 : exp)
yydebug: after reduction, shifting from state 0 to state 60
yydebug: state 60, reducing by rule 1 (start : exp1)
yydebug: after reduction, shifting from state 0 to state 58
$220 = 23
(gdb) FAIL: gdb.base/exprs.exp: print with debugging
...

The test fails because it's not finding the string "Starting parse".

In this case, the .y files are processed used byacc.  I suppose the testcase
matches the case that bison is used.

Fix this by grepping for something more generic: shift or Shift.

Tested on x86_64-linux and x86_64-freebsd.