Fix POSIX-isms in gdb.base/shell.exp

Fix POSIX-isms in gdb.base/shell.exp

Some recent tests added to gdb.base/shell.exp have been failing on
Windows host due to assumptions that the shell is a POSIX variant.  On
Windows, GDB uses CMD.EXE via the system() call to run shell commands
instead.

There seems to be no obvious CMD.EXE equivalent for "kill -2 $$" to
signal the shell process, so this patch skips those tests on Windows
host.  The second problem addressed here is that CMD.EXE only
recognizes double quotes, not single quotes; that change can be made
unconditionally since POSIX shells recognize double quotes as well.

2020-07-16  Sandra Loosemore  <sandra@codesourcery.com>

	gdb/testsuite/
	* gdb.base/shell.exp: Skip pipe tests dependent on sh on Windows host.
	Use double quotes instead of single quotes.