[gdb/testsuite] Make gdb.base/solib-search.exp more robust

[gdb/testsuite] Make gdb.base/solib-search.exp more robust

On aarch64-linux, with gcc 13.2.1, I run into:
...
 (gdb) backtrace^M
 #0  break_here () at solib-search.c:30^M
 #1  0x0000fffff7f20194 in lib2_func4 () at solib-search-lib2.c:50^M
 #2  0x0000fffff7f70194 in lib1_func3 () at solib-search-lib1.c:50^M
 #3  0x0000fffff7f20174 in lib2_func2 () at solib-search-lib2.c:30^M
 #4  0x0000fffff7f70174 in lib1_func1 () at solib-search-lib1.c:30^M
 #5  0x00000000004101b4 in main () at solib-search.c:23^M
 (gdb) PASS: gdb.base/solib-search.exp: \
   backtrace (with wrong libs) (data collection)
 FAIL: gdb.base/solib-search.exp: backtrace (with wrong libs)
...

The FAIL is generated by this code in the test-case:
...
    if { $expect_fail } {
	# If the backtrace output is correct the test isn't sufficiently
	# testing what it should.
	if { $count == $total_expected } {
	    set fail 1
	}
...

The test-case:
- builds two versions of two shared libs, a "right" and "wrong" version, the
  difference being an additional dummy function (called spacer function),
- uses the "right" version to generate a core file,
- uses the "wrong" version to interpret the core file, and
- generates a backtrace.

The intent is that the backtrace is incorrect due to using the "wrong"
version, but actually it's correct.  This is because the spacer functions
aren't large enough.

Fix this by increasing the size of the spacer functions by adding a dummy
loop, after which we have, as expected, an incorrect backtrace:
...
 (gdb) backtrace^M
 #0  break_here () at solib-search.c:30^M
 #1  0x0000fffff7f201c0 in ?? ()^M
 #2  0x0000fffff7f20174 in lib2_func2 () at solib-search-lib2.c:30^M
 #3  0x0000fffff7f20174 in lib2_func2 () at solib-search-lib2.c:30^M
 #4  0x0000fffff7f70174 in lib1_func1 () at solib-search-lib1.c:30^M
 #5  0x00000000004101b4 in main () at solib-search.c:23^M
 (gdb) PASS: gdb.base/solib-search.exp: \
   backtrace (with wrong libs) (data collection)
 PASS: gdb.base/solib-search.exp: backtrace (with wrong libs)
...

Tested on aarch64-linux.