test_runner: always make spec the default reporter

This is a breaking change. Prior to this commit, the test_runner
defaulted to the spec reporter if using a TTY, and the TAP
reporter otherwise. This commit makes spec the default reporter
unconditionally. TAP output is still available via the
--test-reporter=tap CLI flag.

Fixes: nodejs#54540

doc/api/test.md

@@ -1000,12 +1000,13 @@ flags for the test runner to use a specific reporter.
 
 The following built-reporters are supported:
 
+* `spec`
+  The `spec` reporter outputs the test results in a human-readable format. This
+  is the default reporter.
+
 * `tap`
   The `tap` reporter outputs the test results in the [TAP][] format.
 
-* `spec`
-  The `spec` reporter outputs the test results in a human-readable format.
-
 * `dot`
   The `dot` reporter outputs the test results in a compact format,
   where each passing test is represented by a `.`,
@@ -1018,9 +1019,6 @@ The following built-reporters are supported:
   The `lcov` reporter outputs test coverage when used with the
   [`--experimental-test-coverage`][] flag.
 
-When `stdout` is a [TTY][], the `spec` reporter is used by default.
-Otherwise, the `tap` reporter is used by default.
-
 The exact output of these reporters is subject to change between versions of
 Node.js, and should not be relied on programmatically. If programmatic access
 to the test runner's output is required, use the events emitted by the
@@ -3505,7 +3503,6 @@ added:
 Can be used to abort test subtasks when the test has been aborted.
 
 [TAP]: https://testanything.org/
-[TTY]: tty.md
 [`--experimental-test-coverage`]: cli.md#--experimental-test-coverage
 [`--experimental-test-snapshots`]: cli.md#--experimental-test-snapshots
 [`--import`]: cli.md#--importmodule

lib/internal/test_runner/utils.js

@@ -119,7 +119,7 @@ const kBuiltinReporters = new SafeMap([
   ['lcov', 'internal/test_runner/reporter/lcov'],
 ]);
 
-const kDefaultReporter = process.stdout.isTTY ? 'spec' : 'tap';
+const kDefaultReporter = 'spec';
 const kDefaultDestination = 'stdout';
 
 function tryBuiltinReporter(name) {