Simplify output handling in unit tests

[rosenhouse]

• Unit tests respect a new VERBOSE environment variable, which ensures output goes to stdout/stderr.

• When VERBOSE is unset, or set to 0 or false, then:

  • stdout will show only 1 status line per test
  • stderr will be silent, except for error messages

• When VERBOSE is set to any other value, then library and unit test info messages go to stdout.

• When a unit test fails, the user will see a suggestion to set VERBOSE=1 to see all the detailed output. (TODO: the suggestion doesn't appear on a crash, but that'd be nice)

• Library log lines from unit tests no longer go to /tmp/unit_test.std[out,err]

• Platform_default_log_handle and Platform_error_log_handle are no longer part of the public API surface. Applications (and tests that mimic apps) must use platform_set_log_streams() instead (see #487)

• Cleans up some tests to avoid a direct dependency on platform.h:

  • Removes some references to Platform_*_log_handle globals and instead calls new platform_set_log_streams()
  • Removes some calls to platform_default_log() and instead use a dedicated CTEST_LOG_INFO() macro.
  • This isn't complete, but is a start

Fixes #288

---

Here's a side-by-side comparison, running in quiet mode (default) on the left, and with VERBOSE=1 on the right.

and here's what it looks like when a test fails and you haven't set VERBOSE yet:

[netlify[bot]]

✅ Deploy Preview for splinterdb canceled.

Name	Link
🔨 Latest commit	5d3e22407871dc98cb705e78a03af5110ecf0211
🔍 Latest deploy log	https://app.netlify.com/sites/splinterdb/deploys/638faf8d2fd2270008975134

[rosenhouse]

@gapisback Thanks for the review. I've tried to improve the "new test author experience" part by updating splinter_test as much as was practical in a short-time, and leaving a note up top to caution against following patterns there. And then there's at least a clearer distinction between "unit" versus "functional".

And yes, let's merge this as-is, and we'll defer the flag thing to later.