Throw useful error on bad docs snippets (#25389)

You can continue a test started in a previous snippet by marking the
next snippet with `// TEST[continued]`. The trouble is, if you mark the
first snippet in a file with `// TEST[continued]` you'd get difficult
to predict behavior because you'd continue the test started in another
file. This will usually create a test that fails the build but it isn't
easy to track down what you did wrong. This commit catches this
scenario up front and fails the build with a useful error message.

Similarly, if you put `// TEST[continued]` directly after a
`// TESTSETUP` section then the docs tests will fail to run but the
error message did not point you to the `// TEST[continued]` snippet.
This commit catches this scenario up front as well and fails the build
with a useful error message.

0 files changed, 0 insertions, 0 deletions