btrace, test: avoid gdb.btrace/vdso.exp fail

btrace, test: avoid gdb.btrace/vdso.exp fail

The vdso.exp test checks that we can access the VDSO memory when replaying.

Depending on the line information generated by the compiler, runto_main may run
to the line marked with bp.1 or stop before that line.  The test incorrectly
assumes that it will always run to the marked line and fails if it doesn't.

The test does not really care about what is traced.  It does care that GDB is
replaying when capturing the second disassemble output.

Reflect that in the test by ignoring the output of the stepping and record goto
begin commands and by checking that GDB is actually replaying.

testsuite/
	* gdb.btrace/vdso.c (main): Remove breakpoint markers.
	* gdb.btrace/vdso.exp: Change stepping command to "next" and ignore
	its output.  Ignore the output of "record goto begin" and instead
	check that GDB is replaying.