tools: tests: fix regex patterns for timestamps in gpiomon test cases

tools: tests: fix regex patterns for timestamps in gpiomon test cases

Commit f8850206e160 ("gpio: Switch timestamps to ktime_get_ns()") in the
linux kernel (released in v5.7-rc1) changed the clock used to generate
line events from real-time to monotonic. This has the effect of making
the timestamp values much smaller and it uncovered a bug in regex
patterns used to verify gpiomon output: they don't expect there to be
any whitespace characters in the timestamp part of the line.

Fix it by accepting any number of whitespace chars between the opening
'[' and the first digit of the timestamp.

Fixes: 9c5a6f31ebff ("tests: use GLib for library test cases and bats for gpio-tools")
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>