./test: Refactoring of test script for modularization

This refactoring offers following benefits:

  - Unified way how go test commands are being called (in terms of flags intepretation)
  - Uses standard go mechanisms (like go lists) to find files/packages that are subject for test. The mechanism are module aware.
  - Added instruction how to install tools needed for the tests/checkers.
  - Added colors to the output to make it easier to spot any failure.

Confirmed to work using:
- COVERDIR="./coverage" CPU="4" RACE=false COVER=false PASSES="build build_cov cov" ./test
- CPU="4" RACE=false COVER=false PASSES="e2e functional integration" ./test
- COVERDIR="./coverage" COVER="false" CPU="4" RACE="false" PASSES="fmt build unit build_cov integration e2e integration_e2e grpcproxy cov" ./test
- PASSES=unit PKG=./wal TIMEOUT=1m ./test
- PASSES=integration PKG=./clientv3 TIMEOUT=1m ./test
- PASSES=unit PKG=./wal TESTCASE=TestNew TIMEOUT=1m ./test
- PASSES=unit PKG=./wal TESTCASE="\bTestNew\b" TIMEOUT=1m ./test
- PASSES=integration PKG=./client/integration TESTCASE="\bTestV2NoRetryEOF\b" TIMEOUT=1m ./test
- COVERDIR=coverage PASSES="build_cov cov" ./test

.gitignore

@@ -17,5 +17,6 @@ hack/tls-setup/certs
 /contrib/raftexample/raftexample
 /contrib/raftexample/raftexample-*
 /vendor
+/tests/e2e/default.proxy
 
 *.bak