ktesting: fix vmodule support

[pohly]

What this PR does / why we need it:

Stack unwinding to determine the caller must consider the number of call levels to skip over in logr itself. This only affected the Enabled check and thus usage of the vmodule parameter, the actual caller then gets printed by testing.T.

This also leads to a small inconsistency: the enabled check is unaware of the testing.T.Helper calls and can only check the direct caller, without skipping helpers, while the actual log output then skips the helper. There's no good fix for this, testing.T would need an API for delegating the stack unwinding to the caller (would also be useful to log a PC supplied via slog...).

Which issue(s) this PR fixes:

Fixes #420

Special notes for your reviewer:

One line change, multi-line unit test...

Release note:

ktesting: -testing.vmodule was off by one in the call stack when checking whether log ouput should be printed.

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[pohly]

/assign @harshanarayana