test: Avoid intermittent timeout in feature_assumevalid.py #26651

Currently the test will spin up p2p connections in the beginning, then announce the headers to all nodes, but only send the blocks sequentially. This takes a long time, so when getting to the last node, it will have already timed out, while node1 is busy eating blocks. Example:

 node2 2022-12-06T19:31:35.419291Z [msghand] [net_processing.cpp:5783] [SendMessages] [net] Requesting block 2cfdb317b3b901f79e2d4f96339d0c0dffd8ef2685d324f62ab0e2fa3402430e (1) peer=0
 node2 2022-12-06T19:31:35.424784Z [msghand] [net.cpp:2776] [PushMessage] [net] sending getdata (577 bytes) peer=0 
[...]
 node2 2022-12-06T19:41:35.423257Z [msghand] [net_processing.cpp:5729] [SendMessages] Timeout downloading block 2cfdb317b3b901f79e2d4f96339d0c0dffd8ef2685d324f62ab0e2fa3402430e from peer=0, disconnecting  
 node1 2022-12-06T19:41:35.438706Z [msghand] [net_processing.cpp:5783] [SendMessages] [net] Requesting block 6575919043306ed309014d0bd79814b4fab8afaa281e026d8cc3a1c4c2336fbc (1748) peer=0
 node2 2022-12-06T19:41:35.521253Z [net] [net.cpp:573] [CloseSocketDisconnect] [net] disconnecting peer=0 
 node2 2022-12-06T19:41:35.630417Z [net] [net_processing.cpp:1532] [FinalizeNode] [net] Cleared nodestate for peer=0  

Fix this by only spinning up the p2p connection right before they are needed.

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ACK fa34e5f3d35c81bb4376337f4f76c3d501bca742 (jamesob/ackr/26651.1.MarcoFalke.test_avoid_intermittent)

Good fix; ran locally and verified equivalence with old test.

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ACK fa34e5f3d35c81bb4376337f4f76c3d501bca742 ([`jamesob/ackr/26651.1.MarcoFalke.test_avoid_intermittent`](https://github.com/jamesob/bitcoin/tree/ackr/26651.1.MarcoFalke.test_avoid_intermittent))

Good fix; ran locally and verified equivalence with old test.
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<details><summary>Show platform data</summary> <p>

Tested on Linux-6.0.11-arch1-1-x86_64-with-glibc2.36

Configured with ./configure --without-bdb 'CXXFLAGS=-fPIE -pipe -O2 -g -Wthread-safety-analysis -Wall -Werror=sign-compare ' --disable-gui-tests --disable-fuzz-binary --enable-wallet --enable-debug --with-daemon --enable-natpmp-default CXX=/usr/bin/clang++ CC=/usr/bin/clang PYTHONPATH= --disable-shared --with-pic --enable-benchmark=no --enable-module-recovery --enable-module-schnorrsig --disable-shared --with-pic --enable-benchmark=no --enable-module-recovery --enable-module-schnorrsig --no-create --no-recursion

Compiled with /usr/bin/ccache /usr/bin/clang++ -std=c++17 -mavx -mavx2 -mpclmul -fPIE -pipe -O2 -g -Wthread-safety-analysis -Wall -Werror=sign-compare  -O0 -g3 -ftrapv -fdebug-prefix-map=$(abs_top_srcdir)=.  -Wstack-protector -fstack-protector-all -fcf-protection=full -fstack-clash-protection -msse4.1 -msse4.2 -msse4 -msha  i

Compiler version: clang version 14.0.6

</p></details>