gdb: Fix gdb.python/py-record-btrace.exp test

My previous patch

commit 8958aefd (HEAD)
Author: Felix Willgerodt <felix.willgerodt@intel.com>
Date:   Mon Feb 25 15:30:29 2019 +0100

    python: Add clear() to gdb.Record.

exposed a clear function for btrace data in python and added some tests
for it.  That caused a regression (PR 32086) when recording with bts.

This is reproducible even without my patch, when adding
"maintenance btrace clear" to the test.

When comparing the instructions that get recorded in both cases, the traces
are almost identical, just that the first 3 instructions are missing.

Before clear:

(gdb) record instruction-history 1,100
1	   0x0000555555555163 <main+12>:	movl   $0x0,-0x4(%rbp)
2	   0x000055555555516a <main+19>:	movl   $0x0,-0x8(%rbp)
3	   0x0000555555555171 <main+26>:	jmp    0x555555555184 <main+45>
4	   0x0000555555555184 <main+45>:	cmpl   $0x63,-0x4(%rbp)
5	   0x0000555555555188 <main+49>:	jle    0x555555555173 <main+28>
6	   0x0000555555555173 <main+28>:	mov    -0x8(%rbp),%eax
7	   0x0000555555555176 <main+31>:	mov    %eax,%edi
...

After clear:

(gdb) record instruction-history 1,100
1	   0x0000555555555184 <main+45>:	cmpl   $0x63,-0x4(%rbp)
2	   0x0000555555555188 <main+49>:	jle    0x555555555173 <main+28>
3	   0x0000555555555173 <main+28>:	mov    -0x8(%rbp),%eax
4	   0x0000555555555176 <main+31>:	mov    %eax,%edi
...

The GDB manual describes this behaviour already:

	maint btrace clear
	Discard the branch trace data. The data will be fetched anew and
	the branch trace will be recomputed when needed.

	This implicitly truncates the branch trace to a single branch trace
	buffer. When updating branch trace incrementally, the branch trace
	available to GDB may be bigger than a single branch trace buffer.

The test with BTS is updating the recorded trace incrementally.  After the
clear, the buffer of raw trace data available is not enough to recompute the
whole trace as it was before the clear(), and the first 3 instructions are
missing.

As increasing the buffer size for BTS didn't help, I propose to fix the test
by moving the testing of clear to the end of the test.

Approved-By: Tom de Vries <tdevries@suse.de>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32086