gdb/testsuite: Disable some tests that are broken on remote Windows host

These testcases assume host==build or that the remote host has a Posix
shell to run commands in.  Don't try to run them if that's not the case.

Reviewed-By: Tom Tromey <tom@tromey.com>
Approved-By: Tom Tromey <tom@tromey.com>

gdb/testsuite/gdb.base/bad-file.exp

@@ -20,6 +20,9 @@
 # print different text then the plan is to update the expected text
 # instead of making this test linux-only or some such.
 
+# The pathnames and shell commands in this test don't work on Windows host.
+require {!ishost *-*-mingw*}
+
 # There is no such file, but we still use the normal mechanism to pick
 # its name and path.
 standard_testfile

gdb/testsuite/gdb.base/early-init-file.exp

@@ -15,6 +15,9 @@
 
 # Test GDB's early init file mechanism.
 
+# Test assumes host == build.
+require {!is_remote host}
+
 standard_testfile
 
 # Compile the test executable.

gdb/testsuite/gdb.base/empty-host-env-vars.exp

@@ -13,6 +13,9 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+# This test assumes host == build.
+require {!is_remote host}
+
 # GDB reads some environment variables on startup, make sure it behaves
 # correctly if these variables are defined but empty.