tests: add --only=<test>[/<method>] flag

Simplify running a single test (or even a single method from a single
test), which is useful when developing or debugging a test. Instead of
`brew test TEST=test_diagnostic.rb TESTOPTS=--name=test_check_DYLD_vars`
it is now possible to run `brew tests --only=diagnostic/check_DYLD_vars`
to get the same effect. Similarly, `brew tests --only=diagnostic` maps
to `brew test TEST=test_diagnostic.rb`.

Closes Homebrew/homebrew#48279.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Library/Homebrew/cmd/tests.rb

@@ -27,6 +27,11 @@ module Homebrew
 
       args = []
       args << "--trace" if ARGV.include? "--trace"
+      if ARGV.value("only")
+        test_name, test_method = ARGV.value("only").split("/", 2)
+        args << "TEST=test_#{test_name}.rb"
+        args << "TESTOPTS=--name=test_#{test_method}" if test_method
+      end
       args += ARGV.named.select { |v| v[/^TEST(OPTS)?=/] }
       system "bundle", "exec", "rake", "test", *args