Add GO_TEST_EXEC and TESTTAGS to use gotestsum on CI

[pose]

Fixes #2106.

Context

We previously used gotestfmt to improve test output readability on CI, but it was removed because it was mis-attributing logs from one test to another and not reporting logs when a panic was encountered — making failures harder to diagnose rather than easier.

gotestsum is a more robust alternative. This PR introduces two Makefile variables:

• GO_TEST_EXEC ?= go test — set to "gotestsum --" on CI via the workflow env: block
• TESTTAGS ?= all — overridden per-language in the non-sharded matrix path

Why the previous approach didn't work

The first attempt set GO_TEST_EXEC in the workflow env and relied on Make's ?= to pick it up. This worked for the sharded path (which calls make test), but not the non-sharded path, which called go test directly in the workflow YAML — bypassing the Makefile entirely. awsx uses the non-sharded path, so GO_TEST_EXEC was never consulted.

This approach

Move all test invocations through make so GO_TEST_EXEC is always honoured:

• Non-sharded steps now call make TESTTAGS=<language> GOTESTARGS=<flags> test instead of direct go test
• Sharded path already went through make test and continues to do so

This also aligns with the strategic goal of keeping workflows "dumb" — test logic lives in the Makefile, not scattered across workflow YAML.

Also installs gotestsum via aqua in mise.test.toml so it is available in test workflow environments.

Test plan

• make test-go test-providers passes — all 15 test providers regenerate and pass actionlint
• Verify gotestsum output on a real provider CI run

🤖 Generated with Claude Code

[pose]

Sorry @corymhall , I thought I merged the previous PR already. I am updating the previous PR!