t9300: do not run --cat-blob-fd related tests on MinGW

As diagnosed by Johannes Sixt, msys.dll does not hand through file
descriptors > 2 to child processes, so these test cases cannot passes when
run through an MSys bash.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9300-fast-import.sh

@@ -2237,7 +2237,7 @@ test_expect_success 'R: cat-blob-fd must be a nonnegative integer' '
 	test_must_fail git fast-import --cat-blob-fd=-1 </dev/null
 '
 
-test_expect_success 'R: print old blob' '
+test_expect_success NOT_MINGW 'R: print old blob' '
 	blob=$(echo "yes it can" | git hash-object -w --stdin) &&
 	cat >expect <<-EOF &&
 	${blob} blob 11
@@ -2249,7 +2249,7 @@ test_expect_success 'R: print old blob' '
 	test_cmp expect actual
 '
 
-test_expect_success 'R: in-stream cat-blob-fd not respected' '
+test_expect_success NOT_MINGW 'R: in-stream cat-blob-fd not respected' '
 	echo hello >greeting &&
 	blob=$(git hash-object -w greeting) &&
 	cat >expect <<-EOF &&
@@ -2270,7 +2270,7 @@ test_expect_success 'R: in-stream cat-blob-fd not respected' '
 	test_cmp expect actual.1
 '
 
-test_expect_success 'R: print new blob' '
+test_expect_success NOT_MINGW 'R: print new blob' '
 	blob=$(echo "yep yep yep" | git hash-object --stdin) &&
 	cat >expect <<-EOF &&
 	${blob} blob 12
@@ -2288,7 +2288,7 @@ test_expect_success 'R: print new blob' '
 	test_cmp expect actual
 '
 
-test_expect_success 'R: print new blob by sha1' '
+test_expect_success NOT_MINGW 'R: print new blob by sha1' '
 	blob=$(echo "a new blob named by sha1" | git hash-object --stdin) &&
 	cat >expect <<-EOF &&
 	${blob} blob 25