[BUGFIX] concatenateCSS may produce invalid css

css-files with statements like @import might become corrupted. This is due
a wrong regex in case no quotes (single or double) are provided.

Adjust regex and add testcases.

Resolves: #50491
Releases: 6.2, 6.1
Change-Id: I8c35be97147da51e3cfc4be6de114f3c19c1abca
Reviewed-on: https://review.typo3.org/22623
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind