Test point map for BBC micro:bit
The testpoint map is available in this PR, which seems to be stalled on getting merged (which would explain why you haven't found it).



This image is Copyright BBC, CC-CY-4.0

The signal names are as follows, according to the schematic.

 1 TMS  -- Debug interface for KL26
 2 TDO  --  "
 3 TDI  --  "
 4 TCLK --  "
 5 RST* --  "
 6 TGT_SWDIO
 7 TGT_SWCLK
 8 KL26 XTAL -- has 16MHz sine wave
 9 GND
10 VBUS_IF -- carries 5V when USB plugged in
11 USB-D+
12 USB-D-
13 VBAT
14 +V_TGT
15 TGT_TX -- UART transmit from nRF51
16 +3.3V_IF
17 TGT_RX -- UART receive from nRF51
18 VBAT
19 GND
20 TGT_RESET
21 GND
22 GND


In particular, the two UART lines are accessible at TP15 and TP17.