gdb/testsuite: avoid reading files through the remote protocol in gdb.server/*.exp

gdb/testsuite: avoid reading files through the remote protocol in gdb.server/*.exp

When I run some tests in gdb.server (fox example
gdb.server/ext-attach.exp) on Ubuntu 20.04 with separate debug info for
glibc installed, they often time out.  This is because GDB reads the
debug info through the remote protocol which is particularly slow:

    attach 316937
    Attaching to program: /home/smarchi/build/binutils-gdb-all-targets/gdb/testsuite/outputs/gdb.server/ext-attach/ext-attach, process 316937
    Reading /lib/x86_64-linux-gnu/libc.so.6 from remote target...
    warning: File transfers from remote targets can be slow. Use "set sysroot" to access files locally instead.
    Reading /lib64/ld-linux-x86-64.so.2 from remote target...
    Reading symbols from target:/lib/x86_64-linux-gnu/libc.so.6...
    Reading /lib/x86_64-linux-gnu/libc-2.31.so from remote target...
    Reading /lib/x86_64-linux-gnu/.debug/libc-2.31.so from remote target...
    Reading /usr/lib/debug//lib/x86_64-linux-gnu/libc-2.31.so from remote target...
    FAIL: gdb.server/ext-attach.exp: attach to remote program 1 (timeout)

This is avoided in gdbserver boards by adding "set sysroot" to GDBFLAGS
(see boards/local-board.exp), which makes GDB read files from the local
filesystem.  But gdb.server tests spawn GDBserver directly, so are ran
even when using the default unix board, where the "set sysroot" isn't
used.

Modify these tests to append "set sysroot" to the GDBFLAGS, a bit like
lib/local-board.exp does.

One special case is gdb.server/sysroot.exp, whose intent is to test
different "set sysroot" values.  For this one, increase the timeout when
testing the "target:" sysroot.

gdb/testsuite/ChangeLog:

	* gdb.server/abspath.exp: Append "set sysroot" to GDBFLAGS.
	* gdb.server/connect-without-multi-process.exp: Likewise.
	* gdb.server/exit-multiple-threads.exp: Likewise.
	* gdb.server/ext-attach.exp: Likewise.
	* gdb.server/ext-restart.exp: Likewise.
	* gdb.server/ext-run.exp: Likewise.
	* gdb.server/ext-wrapper.exp: Likewise.
	* gdb.server/multi-ui-errors.exp: Likewise.
	* gdb.server/no-thread-db.exp: Likewise.
	* gdb.server/reconnect-ctrl-c.exp: Likewise.
	* gdb.server/run-without-local-binary.exp: Likewise.
	* gdb.server/server-kill.exp: Likewise.
	* gdb.server/server-run.exp: Likewise.
	* gdb.server/solib-list.exp: Likewise.
	* gdb.server/stop-reply-no-thread.exp: Likewise.
	* gdb.server/wrapper.exp: Likewise.
	* gdb.server/sysroot.exp: Increase timeout when testing the
	target: sysroot.

Change-Id: I7451bcc737f90e2cd0b977e9f09da3710774b0bf