bts: Introduce test TC_tx_power_start_ramp_up_bcch

This test verifies power ramping (up) is working fine during BTS
startup.

config files are updated to make sense:
* "nominal power" in osmo-bsc.cfg reflects correct default nominal tx
  power of fake_trx.
* "osmotrx tx-attenuation" in osmo-bts.cfg is removed to let osmo-bts
  use the value received through OML (max_power_red 20).
* "power-ramp step-size" in osmo-bts.cfg is increased to speed up the
  test. There's no good reason to keep it lower.

Change-Id: Ieb7444c6312bbeab64da2732393b3facf3e1f003
diff --git a/bts/osmo-bts.cfg b/bts/osmo-bts.cfg
index a9867bf..c2cd0bd 100644
--- a/bts/osmo-bts.cfg
+++ b/bts/osmo-bts.cfg
@@ -28,7 +28,6 @@
  osmotrx rts-advance 5
  instance 0
   osmotrx rx-gain 10
-  osmotrx tx-attenuation 0
 bts 0
  band DCS1800
  ipa unit-id 1234 0
@@ -49,7 +48,7 @@
  pcu-socket /tmp/pcu_sock
  trx 0
   power-ramp max-initial 0 mdBm
-  power-ramp step-size 2000 mdB
+  power-ramp step-size 8000 mdB
   power-ramp step-interval 1
   ms-power-control dsp
   phy 0 instance 0