What is it called when soldiers stick/leans to the wall before breaching?

The term of the art for what you're describing is the stack.

For instance, this DoD photo describes what the soldiers are doing as

Stack Formation: Paratroopers move along a wall in a stack formation before preparing to clear a building during Exercise Sky Soldier 16 at Chinchilla training area in Albacete, Spain, Feb. 29, 2016.

This reference weighs pros and cons of using variously sized stacks for Close Quarters Battle (CQB).

The act of forming up in this manner (typically outside of a door to a room to be entered, or along a wall before crossing a danger area) is often referred to as stacking up along the wall.

(Your title is asking for a word to described soldiers and policemen all lining up; however, your sample sentence is about a single soldier doing more than just leaning against a wall.)

If you want to emphasize that soldiers are behind each other, one-person deep, when facing the enemy, they are in, or getting in, (a) single file (against a wall).

single file (n.)

A row of persons, animals, or things arranged one behind the other m-w

If they are basically behind one another (but not necessarily in single-file) so not everyone is facing fire initially, you can say they are

in enfilade

A formation or position is "in enfilade" if weapons fire can be directed along its longest axis. artsandculture.google

(Wikipedia illustration)

Origin Early 18th century (denoting the position of a military post commanding the length of a line): from French, from enfiler ‘thread on a string, pierce from end to end’, from en- ‘in, on’ + fil ‘thread’. Lexico

Columns approaching the gun and lines engaged from a position on their flanks are said to be taken in enfilade. Troops taken in enfilade at short ranges are ordinarily annihilated. US Navy; Landing-Force Manual (1927)

A formation or position is “in enfilade” if weapons fire can be directed along its longest axis. For instance, a trench is enfiladed if the opponent can fire down the length of the trench. A column of marching troops is enfiladed if if fired on from the front or rear such that the projectiles travel the length of the columns. A rank or line of advancing troops is enfiladed if fired on from the side (flank). John Shively; Profiles in Survival (2012)