Disable parts of gdb.base/source-dir.exp on remote host

Disable parts of gdb.base/source-dir.exp on remote host

One set of tests in this file does a lot of complicated directory
manipulations to force a specific DW_AT_comp_dir format and gdb
directory search path.  As it's written, everything assumes host ==
build, and it does not seem to me that there is any obvious way to
rewrite this so it will work in general on remote host.  For instance,
our harness for testing on remote Windows host normally does all
compilation and GDB execution in $cwd using relative pathnames and I'm
not sure all these directory tricks would set up the scenario it's
trying to test even if they were correctly performed on host rather
than build.  So I think it's reasonable just to disable this on remote
host instead.

I also noted that it's using the wrong search path syntax for Windows
host in the "set directories" command and conditionalized that while I
was looking at it.  That's a necessary fix to make this work in a
situation where host == build and it's Windows, but I'm not actually
set up to test that it's sufficient, too.

2020-06-22  Sandra Loosemore  <sandra@codesourcery.com>

	gdb/testsuite/
	* gdb.base/source-dir.exp (test_truncated_comp_dir): Skip on
	remote host.  Fix search path syntax on Windows host.