socket: 0.870
network: 0.841
graphic: 0.836
debug: 0.797
peripherals: 0.779
hypervisor: 0.769
device: 0.755
performance: 0.751
PID: 0.742
mistranslation: 0.727
architecture: 0.703
virtual: 0.689
kernel: 0.644
ppc: 0.644
VMM: 0.634
vnc: 0.609
files: 0.594
risc-v: 0.580
permissions: 0.571
TCG: 0.544
user-level: 0.502
boot: 0.477
semantic: 0.473
KVM: 0.446
assembly: 0.427
x86: 0.425
arm: 0.410
register: 0.358
i386: 0.329

NBD tests  use hardcoded port 10810

QEMU v3.1.0

$ ./configure --block-drv-rw-whitelist=qcow2,raw,file,host_device,nbd,iscsi,rbd,blkdebug,luks,null-co,nvme,copy-on-read,throttle,vxhs,gluster [...]

$ ./check -v -nbd 001 002 003 004 005 008 009 010 011 021 032 033 045 077 094 104 119 123 132 143 145 147 151 152 162 181 184 194 205 208 218 222
[...]
104         - output mismatch (see 104.out.bad)
--- tests/qemu-iotests/104.out	2018-12-11 17:44:35.000000000 +0000
+++ tests/qemu-iotests/104.out.bad	2019-09-11 11:59:11.822058653 +0000
@@ -6,7 +6,7 @@
 file format: IMGFMT
 virtual size: 1.0K (1024 bytes)
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1234
-image: TEST_DIR/t.IMGFMT
-file format: IMGFMT
-virtual size: 1.5K (1536 bytes)
+Failed to find an available port: Address already in use
+qemu-img: Could not open 'nbd:127.0.0.1:10810': Failed to connect socket: Connection refused
+./common.rc: line 203: kill: (28391) - No such process
 ***done
Failed 1 of 32 tests

Could be due to concurrent builds on the same system:

$ git grep 10810 tests
tests/qemu-iotests/common.filter:125:        -e 's#nbd:127.0.0.1:10810#TEST_DIR/t.IMGFMT#g' \
tests/qemu-iotests/common.filter:161:        -e 's#nbd://127.0.0.1:10810$#TEST_DIR/t.IMGFMT#g' \
tests/qemu-iotests/common.rc:140:        TEST_IMG="$DRIVER,file.driver=nbd,file.host=127.0.0.1,file.port=10810"
tests/qemu-iotests/common.rc:156:        TEST_IMG="nbd:127.0.0.1:10810"
tests/qemu-iotests/common.rc:276:        eval "$QEMU_NBD -v -t -b 127.0.0.1 -p 10810 -f $IMGFMT -e 42 -x '' $TEST_IMG_FILE >/dev/null &"

This should have been fixed by this commit here:
https://git.qemu.org/?p=qemu.git;a=commitdiff;h=f3923a72f199b2c63747a7