Clean test_{runner,scalafmt_helper}, add diagnostic env vars, and improve test_cross_build performance

[mbland]

Description

Cleans up some of the formatting of test/shell/test_{runner,scalafmt_helper}.sh and adds the RULES_SCALA_TEST_{ONLY,VERBOSE} environment variables. Improves test_cross_build.sh performance signficantly.

Defining RULES_SCALA_TEST_ONLYwill cause only one specific test case to run, with full Bazel output. This is helpful for pinpointing and working on a specific test without hunting for it or recreating its commands.

Defining RULES_SCALA_TEST_VERBOSE will cause all tests to emit full Bazel output. This is more useful when running all the tests from a specific script from test/shell/test_*.sh.

The test_cross_build.sh changes speed up its tests substantially, particularly between runs. Here are the times for two runs before this change; the first is after running bazel clean and bazel shutdown in the test_cross_version directory:

$ /usr/bin/time ./test_cross_build.sh

running test test_cross_build
 Test "test_cross_build" successful (73 sec)
running test test_scalafmt
 Test "test_scalafmt" successful (94 sec)
      167.01 real         0.37 user         0.50 sys

$ /usr/bin/time ./test_cross_build.sh

running test test_cross_build
 Test "test_cross_build" successful (8 sec)
running test test_scalafmt
 Test "test_scalafmt" successful (92 sec)
       99.96 real         0.35 user         0.48 sys

Here are the times for two equivalent runs after this change:

$ /usr/bin/time ./test_cross_build.sh

running test test_cross_build
 Test "test_cross_build" successful (71 sec)
running test test_scalafmt
 Test "test_scalafmt" successful (6 sec)
       77.50 real         0.33 user         0.44 sys

$ /usr/bin/time ./test_cross_build.sh

running test test_cross_build
 Test "test_cross_build" successful (5 sec)
running test test_scalafmt
 Test "test_scalafmt" successful (2 sec)
        7.21 real         0.26 user         0.38 sys

Motivation

The cleanup began as an attempt to get protobuf to stop recompiling so often in my new branch to use the latest abseil-cpp 20240722.0 and protobuf v28.3. It didn't impact that problem, but they were good cleanups regardless. (Though today I learned that SECONDS is a special Bash variable, and marking it local is a mistake that makes all tests look like they ran in zero seconds.)

The test_cross_version.sh speedups were also a result of working on that branch, on which the speedups described above were even more dramatic.

Previously, every bazel run in test_scalafmt would rebuild the entire protobuf library. I thought it was hanging, until I used RULES_SCALA_TEST_ONLY and could see what was happening. Then I used RULES_SCALA_TEST_VERBOSE to see the relationship between bazel invocations in test_cross_build.sh. That led me to remove the bazel clean from test_cross_build and move the bazel shutdown to the end of the script.

As always, this is spawned from work related to #1482, though it is a bit of a side quest.

[simuons]

Thanks!