package.json: exclude scss test files

[XhmikosR]

Before:

npm notice === Tarball Details ===
npm notice name:          bootstrap
npm notice version:       5.3.0-alpha1
npm notice filename:      bootstrap-5.3.0-alpha1.tgz
npm notice package size:  1.7 MB
npm notice unpacked size: 9.5 MB
npm notice shasum:        265605a00a28462aafce59c52082f346efe9df31
npm notice integrity:     sha512-pLr+LPytfJGYz[...]zc5oDOZQzzczg==
npm notice total files:   220

After:

npm notice === Tarball Details ===
npm notice name:          bootstrap
npm notice version:       5.3.0-alpha1
npm notice filename:      bootstrap-5.3.0-alpha1.tgz
npm notice package size:  1.7 MB
npm notice unpacked size: 9.5 MB
npm notice shasum:        6e5f759a822cad4910658db5569607c3fb091e51
npm notice integrity:     sha512-zuPZDyx6SsYdL[...]YOOsT2hww25kA==
npm notice total files:   217

Fixup after #36029